Add back .cc for googletest

Author: WardBrian

make/compiler_flags

@@ -75,7 +75,8 @@ CXX_MINOR := $(word 2,$(subst ., ,$(CXX_VERSION)))
 # this should make Make slightly faster, as well as avoiding some unexpected
 # behavior when there are other files lying around with the same name as
 # Stan models
-.SUFFIXES:
+# We do need to leave some rules in place for things like googletest to function
+.SUFFIXES: .cc 
 
 ################################################################################
 # Set optional compiler flags for performance