curl-compilers.m4: accept -Og and -Ofast GCC flags

-Og, introduced in GCC 4.8, optimizes for debugging experience.
-Ofast, introduced in GCC 4.7, builds on -O3 and enables further
optimizations breaking strict standards compliance.
When specified in CFLAGS, these were always overridden by -O0 or -O2.
Fix this by adding them to flags_opt_all.

Ref: https://gcc.gnu.org/onlinedocs/gcc-4.8.0/gcc/Optimize-Options.html
Ref: https://github.com/curl/curl/pull/1404#issuecomment-296401570
Closes https://github.com/curl/curl/pull/1440