Add TARGET_GCC_VERSION

Default to 4.6, but can overwritten to something else in
prebuilts/gcc/$os/$arch/$arch-linux-android*-$ver

Change-Id: I16f99d642b8ff8e831d392261a0c536041107051
diff --git a/core/config.mk b/core/config.mk
index a894444..06c9074 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -225,6 +225,11 @@
 endif
 TARGET_CPU_ABI2 := $(strip $(TARGET_CPU_ABI2))
 
+# default target GCC version
+ifeq ($(strip $(TARGET_GCC_VERSION)),)
+TARGET_GCC_VERSION := 4.6
+endif
+
 # $(1): os/arch
 define select-android-config-h
 system/core/include/arch/$(1)/AndroidConfig.h