Fix travis ci static automake lib

[mgreter]

Problem:

configure:15694: checking for libsass_version in -lsass
configure:15719: g++ -o conftest -g -O2  -I/home/travis/build/mgreter/sassc/include  -L/home/travis/build/mgreter/sassc/lib conftest.cpp -lsass   >&5
/home/travis/build/mgreter/sassc/lib/libsass.a(plugins.o): In function `Sass::Plugins::load_plugin(std::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)':
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:49: undefined reference to `dlopen'
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:52: undefined reference to `dlsym'
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:57: undefined reference to `dlsym'
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:63: undefined reference to `dlsym'
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:69: undefined reference to `dlsym'
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:89: undefined reference to `dlerror'
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:81: undefined reference to `dlerror'
/home/travis/build/mgreter/sassc/libsass/src/plugins.cpp:82: undefined reference to `dlclose'
collect2: ld returned 1 exit status

Fix:

if test "x$is_mingw32" != "xyes"; then
  dnl The dlopen() function is in the C library for *BSD and in
  dnl libdl on GLIBC-based systems. Windows uses `LoadLibrary`!
  AC_SEARCH_LIBS([dlopen], [dl dld], [], [
    AC_MSG_ERROR([unable to find the dlopen() function])
  ])
fi

[am11]

👍