cogl/configure.ac: check when building for win32
cogl/configure.ac: check when building for win32

This adds a check for when building on win32 so we can skip pkg-config
checks for opengl and can add appropriate flags to
COGL_EXTRA_{LD,C}FLAGS.

djdeath/cogl,gcampax/cogl,spatulasnout/cogl,gcampax/cogl,Distrotech/cogl,collects/cogl,collects/cogl,spatulasnout/cogl,Distrotech/cogl,Distrotech/cogl,djdeath/cogl,Distrotech/cogl,djdeath/cogl-android,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android