Permit opting out of auto-optimization in GCC

[mwinterb]

I don't know if this is what #2526 is attempting to address since there's not enough detail, but old GCC's targeting ARM (and AVR and MSP430) emit a warning on #pragma GCC pop_options, see https://godbolt.org/z/YqrGTGMEq. The ARM and AVR targets are using GCC's older than what fmtlib supports, but I know that ARM targeting GCC 4.8.3 as well as 4.9 based on this bug report also emitted it, but godbolt doesn't have those specific versions.

[vitaut]

Implemented a workaround for the GCC bug in 012cc70. You don't really need to opt out (although you could by defining FMT_PRAGMA).