contrib: Set _CMAKE_TOOLCHAIN_PREFIX

This is required for detecting ar and ranlib properly, required
e.g. when cross compiling for android from OS X.

One can't set CMAKE_AR or CMAKE_RANLIB directly. Normally they
should be set properly automatically within cmake, but those
codepaths seem to be skipped when we set the C/CXX compilers
manually.
Signed-off-by: Martin Storsjö <martin@martin.st>

contrib/src/main.mak

@@ -391,6 +391,9 @@ ifdef HAVE_DARWIN_OS
 	echo "set(CMAKE_C_FLAGS $(CFLAGS))" >> $@
 	echo "set(CMAKE_CXX_FLAGS $(CFLAGS))" >> $@
 	echo "set(CMAKE_LD_FLAGS $(LDFLAGS))" >> $@
+endif
+ifdef HAVE_CROSS_COMPILE
+	echo "set(_CMAKE_TOOLCHAIN_PREFIX $(HOST)-)" >> $@
 endif
 	echo "set(CMAKE_C_COMPILER $(CC))" >> $@
 	echo "set(CMAKE_CXX_COMPILER $(CXX))" >> $@