cmake: c99 isn't really required, only enable compiler extensions.

[sezero]

also make example program to build in c90 mode.

CC: @mmontag

[mmontag]

I asked Copilot for fun. Will merge if you don't see anything surprising here:

The FluidLite repository uses some C99 constructs, as observed from the code snippets. Here are key findings:

1. C99-Specific Features Identified:

   • Usage of inline keyword in various files such as libvorbis-1.3.5/lib/os.h and others.
   • Inclusion of C99 headers like <stdint.h> in multiple files, e.g., libogg-1.3.2/include/ogg/config_types.h.
   • Use of __attribute__((constructor)) in src/fluid_init.c, which is a GCC-specific feature commonly associated with C99 or later.

2. Other Observations:

   • Some files include macros and constructs (#define) that may indicate compatibility with certain C99 features.
   • There are instances of comments using // which are valid in C99 but not in ANSI C (C90).

For full compatibility with ANSI C (C90), these constructs would need to be refactored or replaced. Let me know if you'd like assistance in determining specific changes needed for C90 compatibility!

[sezero]

The dependencies' reliances shouldn't affect fluidlite, and considering the dependency is libvorbis, it was written years ago w/o c99 in mind. I was actually able to build fluidlite using gcc2.95 which says something.

In any case, feel free to ignore this one.

P.S.: You can also drop the c99 thing from y-the cmake script entirely. (tell me if you want such a thing.)