This reverts commit a45a50a482.
Unfortunately gcc dumps argv[0] as the first word of --version, so it is
unreliable for detecting gcc.
In particular `cc --version` and `i686-w64-mingw32-gcc --version` give
wrong results.
A better solution needs to be found -- most likely using C-preprocessing
like autotools does. Revert for now.
e3c58cdfd9