external/firebird: Fix isinf-check for libc++

...which, at least since version 3.9, defines isinf only as a function, not as a
macro, in math.h

Change-Id: I7939d5bebe4b55ac7bd36bb321ad0020352a0e7b

external/firebird/UnpackedTarball_firebird.mk

@@ -21,6 +21,7 @@ $(eval $(call gb_UnpackedTarball_add_patches,firebird,\
 		external/firebird/firebird-gcc6.patch.1 \
 		external/firebird/wnt-dbgutil.patch \
 		external/firebird/debug-gfix-failure.patch \
+		external/firebird/libc++.patch \
 ))
 
 ifeq ($(OS)-$(COM),WNT-MSC)

external/firebird/libc++.patch

+--- src/common/classes/FpeControl.h
++++ src/common/classes/FpeControl.h
+@@ -222,7 +222,7 @@
+ {
+ 	return (!_finite (x) && !isnan(x));
+ }
+-#else
++#elif !defined _LIBCPP_VERSION || _LIBCPP_VERSION < 3900
+ #ifndef isinf
+ template <typename F>
+ inline bool isinf(F x)