Default compiler optimization level for building dev sources

[Óscar Fuentes]

I'm using Emacs 25.0.92 on Windows and it seems remarkably slower than
the previous binary. This one was built with a plain config && make
while the previous one had CFLAGS set to -O3 and other extra
optimization flags.

Maybe the build does not turn on optimizations when it detects that the
sources are on a repo. Sorry for asking here instead of checking it
myself but today I have no access to the sources nor the web.