configure: fix compilation on recent macOS Xcode versions

Back in ccf2d89 ("configure: try to
disable weak linking on OSX") disabling weak symbols was done to prevent
depending on features when building on a newer macOS but targeting an
older one. Fast-forward to Xcode 11.4 and it turns out Apple have broken
this by depending on weak symbols for critical symbols like FD_SET
leading to compilation errors like the following

ld: weak import of symbol '___darwin_check_fd_set_overflow' not supported because of option: -no_weak_imports for architecture x86_64

Other people/projects have been hit by this issue:
- https://openradar.appspot.com/FB7647406
- mono/mono#19393

but Apple have kept this behaviour into XCode 12 so we should adapt.

- Introduce the concept of configure only CFLAGS
- Switch to using -Werror=partial-availability with the above
- Stop logging a message about disabling weak linking

The above should avoid finding features the target platform doesn't have
while allowing weak linking. The name CONFIGURE_CFLAGS is used to remain
similar to QEMU's configure. Fingers crossed this approach stays
supported by Apple...

Fixes axboe#1006

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>

Author: sitsofe

configure

@@ -45,6 +45,7 @@ print_config() {
 
 # Default CFLAGS
 CFLAGS="-D_GNU_SOURCE -include config-host.h $CFLAGS"
+CONFIGURE_CFLAGS="-Werror-implicit-function-declaration"
 BUILD_CFLAGS=""
 
 # Print a helpful header at the top of config.log
@@ -88,14 +89,14 @@ do_cc() {
 }
 
 compile_object() {
-  do_cc $CFLAGS -Werror-implicit-function-declaration -c -o $TMPO $TMPC
+  do_cc $CFLAGS $CONFIGURE_CFLAGS -c -o $TMPO $TMPC
 }
 
 compile_prog() {
   local_cflags="$1"
   local_ldflags="$2 $LIBS"
   echo "Compiling test case $3" >> config.log
-  do_cc $CFLAGS -Werror-implicit-function-declaration $local_cflags -o $TMPE $TMPC $LDFLAGS $local_ldflags
+  do_cc $CFLAGS $CONFIGURE_CFLAGS $local_cflags -o $TMPE $TMPC $LDFLAGS $local_ldflags
 }
 
 feature_not_found() {
@@ -360,16 +361,15 @@ Darwin)
   if test -z "$cpu" && test "$(sysctl -n hw.optional.x86_64)" = "1"; then
     cpu="x86_64"
   fi
-  # Error at compile time linking of weak/partial symbols if possible...
+  # Avoid configure feature detection of features provided by weak symbols
 cat > $TMPC <<EOF
 int main(void)
 {
   return 0;
 }
 EOF
-  if compile_prog "" "-Wl,-no_weak_imports" "disable weak symbols"; then
-    echo "Disabling weak symbols"
-    LDFLAGS="$LDFLAGS -Wl,-no_weak_imports"
+  if compile_prog "" "-Werror=partial-availability" "error on weak symbols"; then
+    CONFIGURE_CFLAGS="$CONFIGURE_CFLAGS -Werror=partial-availability"
   fi
   ;;
 SunOS)