Straighten out the protobuf-cpp vs zlib mess.

The protobuf-cpp CMakefile uses find_package() to detect zlib. For some
reason, CMake decides to add the include path of zlib's headers with
-isystem. This is bad, as it causes us to include <math.h> and friends
in the wrong order.

Pass in CMAKE_NO_SYSTEM_FROM_IMPORTED to CMake to prevent it from adding
-isystem flags.

src/builder.py

@@ -398,6 +398,7 @@ def cmake(self, builddir, sourcedir, args):
             '-DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY',
             '-DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER',
             '-DCMAKE_INSTALL_PREFIX=' + self.get_prefix(),
+            '-DCMAKE_NO_SYSTEM_FROM_IMPORTED=ON',
             '-DCMAKE_PREFIX_PATH=' + self._localbase,
             '-DCMAKE_RANLIB=' + self._tool('ranlib'),
             '-DCMAKE_SYSTEM_NAME=Generic',