makefile: allow .git to be a file or directory to handle submodules

Change test for -e (exists) instead of -d (directory) so that the .git
entry is detected correctly and git version stamp logic runs as in any
other git repo. The .git entry is a file when the repo is a submodules
and possibly other obscure cases like multiple worktrees.

fixes #988

makefile

@@ -273,7 +273,7 @@ ifeq (${WIN32},)  #*nix Environments (&& cygwin)
       OS_CCDEFS += -Wno-deprecated
     endif
   endif
-  ifeq (git-repo,$(shell if ${TEST} -d ./.git; then echo git-repo; fi))
+  ifeq (git-repo,$(shell if ${TEST} -e ./.git; then echo git-repo; fi))
     GIT_PATH=$(strip $(shell which git))
     ifeq (,$(GIT_PATH))
       $(error building using a git repository, but git is not available)