m4: workaround clang/glibc problem with isnan()

When building libvirt with clang we get bogus warnings about
'double' being promoted to 'long double' when calling isnan().

  https://bugzilla.redhat.com/show_bug.cgi?id=1472437

Detect this broken isnan() / compiler combination and disable
the -Wdouble-promotion flag.
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

m4/virt-compile-warnings.m4

@@ -134,6 +134,24 @@ AC_DEFUN([LIBVIRT_COMPILE_WARNINGS],[
         [lv_cv_gcc_wlogical_op_equal_expr_broken=yes])
         CFLAGS="$save_CFLAGS"])
 
+    AC_CACHE_CHECK([whether clang gives bogus warnings for -Wdouble-promotion],
+      [lv_cv_clang_double_promotion_broken], [
+        save_CFLAGS="$CFLAGS"
+        CFLAGS="-O2 -Wdouble-promotion -Werror"
+        AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+          #include <math.h>
+        ]], [[
+          float f = 0.0;
+	  return isnan(f);]])],
+        [lv_cv_clang_double_promotion_broken=no],
+        [lv_cv_clang_double_promotion_broken=yes])
+        CFLAGS="$save_CFLAGS"])
+
+    if test "$lv_cv_clang_double_promotion_broken" = "yes";
+    then
+      dontwarn="$dontwarn -Wdouble-promotion"
+    fi
+
     # We might fundamentally need some of these disabled forever, but
     # ideally we'd turn many of them on
     dontwarn="$dontwarn -Wfloat-equal"