imake: embed GCC's opt_prefix, not prefix
imake: embed GCC's opt_prefix, not prefix

Fixes #110.
Closes #111.

Signed-off-by: Misty De Meo <85d6b3741948526aebc363d97f74b6575b2cc027@gmail.com>

UniqMartin/homebrew-x11,BrewTestBot/homebrew-x11,princeofdarkness76/homebrew-x11