We do not need (should not, at least) strict ISO Standard C (C99) compiler,
and if we insist on it using AC_PROG_CC_STDC instead of AC_PROG_CC we
actually end up with compile options added which force c99 mode and manages
to break compilation with GCC 4.3.0

See http://sunsite.mff.cuni.cz/rawhide20071220-gcc43/multipledef/pidgin-guifications-2.14-2.fc7.log
for the failure, and http://www.redhat.com/archives/fedora-devel-list/2008-January/msg00128.html
for an explanation.