Update git repo check logic in Makefile

Checking whether any submodules are registered is slightly more precise than checking for raw git.

If we aren't in a git repository or that repo has no registered submodules, then the build is definitely created from the archive. Running `git submodule update` won't help in that case.

Author: Vekhir

src/Makefile

@@ -30,21 +30,21 @@ checkghcpkg=$(if $(shell ghc-pkg list --simple-output $(1)),,$(error GHC package
 $(foreach pkg,$(GHC_PKGS),$(call checkghcpkg,$(pkg)))
 
 # Check all submodules are initialized
-in_git_repo=$(shell git rev-parse --is-inside-work-tree 2> /dev/null )
-ifeq ($(in_git_repo),true)
-define SUBMOD_MSG
-Submodule $(1) missing. Initialize with
-    git submodule update --init --recursive
-endef
-checksubmod=$(if $(wildcard $(1)/.git),,$(error $(SUBMOD_MSG)))
-else
+in_git_repo=$(shell git submodule 2> /dev/null )
+ifneq ($(in_git_repo),)
 define SUBMOD_MSG
 Submodule $(1) missing.
     This archive was exported from Git without the source files for
     submodules. You will need to acquire those files or, preferably,
     build instead from a clone of the BSC Git repository
 endef
 checksubmod=$(if $(wildcard $(1)/*),,$(error $(SUBMOD_MSG)))
+else
+define SUBMOD_MSG
+Submodule $(1) missing. Initialize with
+    git submodule update --init --recursive
+endef
+checksubmod=$(if $(wildcard $(1)/.git),,$(error $(SUBMOD_MSG)))
 endif
 $(foreach submod,$(SUBMODS),$(call checksubmod,$(submod)))
 endif # NO_DEPS_CHECKS