r18859: finally worked out what is going wrong with immediate structures. The

r18859: finally worked out what is going wrong with immediate structures. The
problem is that the AC_PROG_CC_C99 macro ends up selecting either
-std=std99 or -std=gnu99 for gcc. Ironically enough, that breaks
constant structure initialisers!

So, simplest solution is to not try that configure test if we know we
are using gcc
(This used to be commit 331435daf3275acaf282c1032c6e9f7dc3e685bb)