Need -lm for Android for the exectuables that fontconfig builds

Said executables will not be used for anything, of course.

We can't use gb_STDLIBS, which also contains -lm after my previous
commit, as fontconfig is C, not C++, and gb_STDLIBS contains
-lgnustl_static, and only our $(CXX), not $(CC), contains the -L that
points to where -lgnustl_static is to be found.

Change-Id: I40c459580f357d913ddc55eae00e16f90f81d510

fontconfig/ExternalProject_fontconfig.mk

@@ -21,7 +21,7 @@ $(eval $(call gb_ExternalProject_register_targets,fontconfig,\
 
 $(call gb_ExternalProject_get_state_target,fontconfig,build) :
 	cd $(EXTERNAL_WORKDIR) \
-	&& $(if $(debug),CFLAGS=-g) ./configure \
+	&& $(if $(debug),CFLAGS=-g) $(if $(filter ANDROID,$(OS)),LIBS="-lm $(LIBS)") ./configure \
 		--disable-shared \
 		--with-arch=arm \
 		--with-expat-includes=$(OUTDIR)/inc/external \