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.