cmake: build in c++11 mode for newer protobufs

[jonchang]

Builds fail with many errors on newer protobuf versions > 3.1, which is no longer packaged in e.g., Homebrew.

c.f. brewsci/homebrew-bio#832

In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/main.cpp:34:
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/alignments.pb.h:23:
In file included from /usr/local/include/google/protobuf/io/coded_stream.h:140:
/usr/local/include/google/protobuf/stubs/common.h:186:17: error: expected expression
  OnShutdownRun([](const void* pp) { delete static_cast<const T*>(pp); }, p);
                ^
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/main.cpp:34:
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/alignments.pb.h:23:
/usr/local/include/google/protobuf/io/coded_stream.h:847:10: error: unknown type name 'constexpr'
  static constexpr int TagSize(uint32 tag) {
         ^
/usr/local/include/google/protobuf/io/coded_stream.h:847:20: error: expected member name or ';' after declaration specifiers
  static constexpr int TagSize(uint32 tag) {
  ~~~~~~~~~~~~~~~~ ^
/usr/local/include/google/protobuf/io/coded_stream.h:695:14: error: use of undeclared identifier 'WriteRawFallback'
      return WriteRawFallback(data, size, ptr);
             ^
/usr/local/include/google/protobuf/io/coded_stream.h:716:46: error: use of undeclared identifier 'TagSize'
            size >= 128 || end_ - ptr + 16 - TagSize(num << 3) - 1 < size)) {
                                             ^
/usr/local/include/google/protobuf/io/coded_stream.h:733:46: error: use of undeclared identifier 'TagSize'
            size >= 128 || end_ - ptr + 16 - TagSize(num << 3) - 1 < size)) {
                                             ^
/usr/local/include/google/protobuf/io/coded_stream.h:786:5: error: unknown type name 'constexpr'
    constexpr auto element_size = sizeof(typename T::value_type);
    ^
/usr/local/include/google/protobuf/io/coded_stream.h:786:15: error: expected unqualified-id
    constexpr auto element_size = sizeof(typename T::value_type);
              ^
/usr/local/include/google/protobuf/io/coded_stream.h:787:28: error: use of undeclared identifier 'element_size'
    auto size = r.size() * element_size;
                           ^
/usr/local/include/google/protobuf/io/coded_stream.h:789:33: error: use of undeclared identifier 'element_size'
    return WriteRawLittleEndian<element_size>(r.data(), static_cast<int>(size),
                                ^
/usr/local/include/google/protobuf/io/coded_stream.h:878:11: error: use of undeclared identifier 'WriteLengthDelim'
    ptr = WriteLengthDelim(num, size, ptr);
          ^
/usr/local/include/google/protobuf/io/coded_stream.h:1048:9: error: delegating constructors are permitted only in C++11
      : CodedOutputStream(stream, true) {}
        ^~~~~~~~~~~~~~~~~
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/main.cpp:34:
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/alignments.pb.h:24:
/usr/local/include/google/protobuf/arena.h:250:31: error: expected unqualified-id
class PROTOBUF_EXPORT alignas(8) Arena final {
                              ^
/usr/local/include/google/protobuf/arena.h:250:31: error: expected ')'
/usr/local/include/google/protobuf/arena.h:250:30: note: to match this '('
class PROTOBUF_EXPORT alignas(8) Arena final {
                             ^
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/main.cpp:34:
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/alignments.pb.h:25:
/usr/local/include/google/protobuf/arenastring.h:154:14: error: member access into incomplete type 'google::protobuf::Arena'
        arena->Own(value);
             ^
/usr/local/include/google/protobuf/arena.h:75:7: note: forward declaration of 'google::protobuf::Arena'
class Arena;    // defined below
      ^
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/main.cpp:34:
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/alignments.pb.h:25:
/usr/local/include/google/protobuf/arenastring.h:372:12: error: incomplete type 'google::protobuf::Arena' named in nested name specifier
    ptr_ = Arena::Create< ::std::string>(arena, *initial_value);
           ^~~~~~~
/usr/local/include/google/protobuf/arena.h:75:7: note: forward declaration of 'google::protobuf::Arena'
class Arena;    // defined below
      ^
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/main.cpp:34:
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/alignments.pb.h:25:
/usr/local/include/google/protobuf/arenastring.h:372:40: error: expected '(' for function-style cast or type construction
    ptr_ = Arena::Create< ::std::string>(arena, *initial_value);
                          ~~~~~~~~~~~~~^
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/main.cpp:34:
In file included from /tmp/express-20191114-21111-xza7wy/eXpress-1.5.2/src/alignments.pb.h:26:
In file included from /usr/local/include/google/protobuf/generated_message_table_driven.h:34:
In file included from /usr/local/include/google/protobuf/map.h:48:
In file included from /usr/local/include/google/protobuf/generated_enum_util.h:36:
/usr/local/include/google/protobuf/message_lite.h:135:3: error: unknown type name 'constexpr'
  constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }
  ^
/usr/local/include/google/protobuf/message_lite.h:135:13: error: expected member name or ';' after declaration specifiers
  constexpr const T& get() const { return reinterpret_cast<const T&>(union_); }
  ~~~~~~~~~ ^
fatal error: too many errors emitted, stopping now [-ferror-limit=]
20 errors generated.
make[2]: *** [src/CMakeFiles/express.dir/main.cpp.o] Error 1
make[2]: *** Waiting for unfinished jobs....
make[1]: *** [src/CMakeFiles/express.dir/all] Error 2
make: *** [all] Error 2