partly revert detection of broken float in configure

commit 80fbaac4cd broke all soft-float
archs, where gcc defines __GCC_IEC_559==0 because rounding modes and
exception flags are not supported. for now, just check for
__FAST_MATH__ as an indication of broken float. this won't detect all
possible misconfigurations but it probably catches the most common
one.

configure

@@ -660,7 +660,7 @@ fi
 # for all packages. On recent GCC we can detect this and error out
 # early rather than producing a seriously-broken math library.
 #
-if trycppif "__FAST_MATH__ || (defined(__GCC_IEC_559) && __GCC_IEC_559==0)" \
+if trycppif "__FAST_MATH__" \
   "$CFLAGS_C99FSE $CPPFLAGS $CFLAGS" ; then
 fail "$0: error: compiler has broken floating point; check CFLAGS"
 fi