Add compiler optimization flags to ceed.pc

[jrwrigh]

See HONEE thread for background.

The idea is that downstream users of libCEED can/should inherit the compilation flags of upstream as they should respond similarly to the same optimization flags.

To do that, we could add a cflags_extra key to ceed.pc to store then, similar to how PETSc does it. And in that we'd just store the OPT variable from the build process.

[jedbrown]

I'm fine with providing this key, though it may be tricky to record the right one. For example, when PETSc --download-libceed, it sets CFLAGS directly, not via the OPT convenience (whose sole purpose is to make it easy to give optimization flags for C, C++, and Fortran in one shot (only works if they have compatible vendors). https://gitlab.com/petsc/petsc/-/blob/main/config/BuildSystem/config/packages/libceed.py?ref_type=heads#L34

[jrwrigh]

Good point. I'm fine with it being CFLAGS instead of OPT. The thought behind OPT is that it just trims down on the intake of flags. CFLAGS seems like the better one for robustness sake.