Increase minimum -std= in Makefile

Hello --

Some old versions of gcc, such as 4.2.1, may fail to build Frotz due to the following combination: (1) an anonymous union in sound_event_t in src/curses/ux_audio.c, (2) -std=c99 in Makefile.

Anonymous unions are not part of C99. They are part of C11. They are also a valid GNU extension to C99. Because of this, the older versions of gcc are perfectly within reason to reject the code if compiled at -std=c99, though it appears that clang and newer versions of gcc do not reject at -std=c99. This issue was discovered by the OpenBSD package build cluster, in which the sparc64 cluster using gcc-4.2.1 failed to build the recently updated frotz package.

This could be fixed by changing the minimum -std= in the Makefile to -std=gnu99. Or the union could be de-anonymized if non gcc/clang compiler support is needed.

Thanks!