configure: make sure LTO does not optimize out the test functions

Fixes trac ticket #5909

Bud-Id: https://bugs.gentoo.org/show_bug.cgi?id=598054
Acked-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
(cherry picked from commit 890eb3d7c4)
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

configure

@@ -1146,10 +1146,16 @@ check_func_headers(){
         for hdr in $headers; do
             print_include $hdr
         done
+        echo "#include <stdint.h>"
         for func in $funcs; do
             echo "long check_$func(void) { return (long) $func; }"
         done
-        echo "int main(void) { return 0; }"
+        echo "int main(void) { int ret = 0;"
+        # LTO could optimize out the test functions without this
+        for func in $funcs; do
+            echo " ret |= ((intptr_t)check_$func) & 0xFFFF;"
+        done
+        echo "return ret; }"
     } | check_ld "cc" "$@" && enable $funcs && enable_safe $headers
 }