firebird: --enable-debug breaks the build with MSVC

With the 2.5.4 upgrade somehow this fails to link fbintl.lib with
unresolved symbols.

Debugging is overrated anyway, let's just disable that for now.

Change-Id: I265c705c10ca7c70baa18232f6f740120d6b397e

external/firebird/ExternalProject_firebird.mk

@@ -50,7 +50,7 @@ $(call gb_ExternalProject_get_state_target,firebird,build):
 			--without-editline \
 			--disable-superserver \
 			--with-system-icu --without-fbsample --without-fbsample-db \
-			$(if $(ENABLE_DEBUG),--enable-debug) \
+			$(if $(filter-out MSC,$(COM)),$(if $(ENABLE_DEBUG),--enable-debug)) \
 			$(if $(CROSS_COMPILING),--build=$(BUILD_PLATFORM) --host=$(HOST_PLATFORM)) \
 			$(if $(DISABLE_DYNLOADING), \
 				--enable-static --disable-shared \