Fix the cached value of ac_cv_c_compiler_gnu [v9.18]

There was an error in AX_PROG_CC_FOR_BUILD macro that cached literal name of the cache variable saved_ac_cv_c_compiler_gnu instead of the value of said variable breaking the consecutive runs of ./configure script with caching enabled.

(cherry picked from commit 4a9f899b)

Closes #3299 (closed)