cpp: Set default visibility of all symbols to hidden

* configure.ac: Add -fvisibility=hidden to GPGME_CPP_CFLAGS if gcc
supports the flag.
* lang/cpp/src/Makefile.am (AM_CPPFLAGS): Add GPGME_CPP_CFLAGS.
* m4/ax_gcc_func_attribute.m4: New.

With this change all defined symbols are hidden by default, so that they
are not exported anymore. All symbols that are part of the ABI and that
shall still be exported are already marked as having default visibility.

The m4 macro was taken from the website mentioned in the License header
of the file.

• GnuPG-bug-id: T5906