Dynamically collect the true git exec path and only if that can not be found, use the default. Variable GIT_EXEC_PATH remains overwritable explicitly when given as a command to 'make'.
diff --git a/Makefile b/Makefile
index 35cf757..7eb22fb 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-GIT_EXEC_PATH=/usr/libexec/git-core
+GIT_EXEC_PATH=`git --exec-path 2>/dev/null || echo /usr/libexec/git-core`
 
 all:
 	@echo "There is only one target here: install"