add configure check for broken gcc 4.9.0 and possibly other versions

this is gcc bug #61144. the broken compiler is detected, but the user
must manually work around it. this is partly to avoid complex logic
for adding workaround CFLAGS and attempting to recheck with them, and
partly for the sake of letting the user know the compiler is broken
(since the workaround will result in less-efficient code production).

some refactoring was also needed to move the check for gcc outside of
the check for whether to build the compiler wrapper.

configure

@@ -125,6 +125,7 @@ debug=no
 warnings=no
 shared=yes
 static=yes
+wrapper=auto
 
 for arg ; do
 case "$arg" in
@@ -199,23 +200,33 @@ exit 1
 fi
 
 #
-# Only build musl-gcc wrapper if toolchain does not already target musl
+# Need to know if the compiler is gcc to decide whether to build the
+# musl-gcc wrapper, and for critical bug detection in some gcc versions.
 #
-if test -z "$wrapper" ; then
 printf "checking whether compiler is gcc... "
 if fnmatch '*gcc\ version*' "$($CC -v 2>&1)" ; then
-echo yes
+cc_is_gcc=yes
+else
+cc_is_gcc=no
+fi
+echo "$cc_is_gcc"
+
+#
+# Only build musl-gcc wrapper if toolchain does not already target musl
+#
+if test "$wrapper" = auto ; then
 printf "checking whether to build musl-gcc wrapper... "
+if test "$cc_is_gcc" = yes ; then
 wrapper=yes
 while read line ; do
 case "$line" in */ld-musl-*) wrapper=no ;; esac
 done <<EOF
 $($CC -dumpspecs)
 EOF
-echo $wrapper
 else
-echo no
+wrapper=no
 fi
+echo "$wrapper"
 fi
 
 
@@ -484,6 +495,27 @@ printf "no\n"
 fail "$0: error: unsupported long double type"
 fi
 
+#
+# Check for known bug in GCC 4.9.0 that results in a broken libc.
+#
+if test "$cc_is_gcc" = yes ; then
+printf "checking for gcc constant folding bug with weak aliases... "
+echo 'static int x = 0;' > "$tmpc"
+echo 'extern int y __attribute__((__weak__, __alias__("x")));' >> "$tmpc"
+echo 'extern int should_appear;' >> "$tmpc"
+echo 'int foo() { return y ? should_appear : 0; }' >> "$tmpc"
+case "$($CC $CFLAGS_C99FSE -I./arch/$ARCH -I./include \
+  $CPPFLAGS $CFLAGS_AUTO $CFLAGS -S -o - "$tmpc" 2>/dev/null)" in
+*should_appear*)
+printf "no\n"
+;;
+*)
+printf "yes\n"
+fail "$0: error: broken compiler; try CFLAGS=-fno-toplevel-reorder"
+;;
+esac
+fi
+
 printf "creating config.mak... "
 
 cmdline=$(quote "$0")