build: allow gtest to be collected

Author: davvid

Makefile.config.example

@@ -5,7 +5,11 @@ uname_S := $(shell uname -s)
 # Example Linux config
 ifeq ($(uname_S),Linux)
 
-    EXTRA_CMAKE_ARGS += -DGTEST_DIR=/usr
+    ifdef RP_gtest
+        EXTRA_CMAKE_ARGS += -DGTEST_DIR=$(RP_gtest)
+    else
+        EXTRA_CMAKE_ARGS += -DGTEST_DIR=/usr
+    endif
 
     ifdef RP_animlib
         EXTRA_CMAKE_ARGS += -DANIMLIB_DIR=$(RP_animlib)