Fix: put auto-detected Makefile path into a variable, but keep
GIT_EXEC_PATH overrideable via Makefile command arguments.
diff --git a/Makefile b/Makefile
index 8b13f2d..11bf22d 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,5 @@
-GIT_EXEC_PATH=`git --exec-path 2>/dev/null || echo /usr/libexec/git-core`
+AUTO_DETECTED_GIT_EXEC_PATH := $(shell git --exec-path 2>/dev/null || echo /usr/libexec/git-core)
+GIT_EXEC_PATH=$(AUTO_DETECTED_GIT_EXEC_PATH)
 
 # files that need mode 755
 EXEC_FILES=git-flow