[build] Bail out early if `git` is not installed

Otherwise typing `make` would fail with this error:

```
$ make
if test -d ".git"; then \
        git submodule sync; \
        git submodule update --init --recursive || exit 1; \
fi
/bin/sh: 2: git: not found
/bin/sh: 3: git: not found
Makefile:12: recipe for target 'update_submodules' failed
make: *** [update_submodules] Error 1
```

1 files changed, 9 insertions, 1 deletions

diff --git a/Makefile b/Makefile
index b40350491f..c13049636e 100644
--- a/Makefile
+++ b/Makefile
@@ -8,12 +8,20 @@ MONO_AOT:=MONO_PATH=$(AOT_DIRECTORIES):$(MONO_PATH) mono --aot --debug
 
 all: update_submodules all-recursive
 
-update_submodules:
+GIT_FOUND = $$(echo $$(which git))
+SYNC_SUBMODULES = \
 	if test -d ".git"; then \
+		if [ "x$(GIT_FOUND)" = "x" ]; then \
+			echo "git not found; please install it first"; \
+			exit 1; \
+		fi; \
 		git submodule sync; \
 		git submodule update --init --recursive || exit 1; \
 	fi
 
+update_submodules:
+	@$(SYNC_SUBMODULES)
+
 top_srcdir=.
 include $(top_srcdir)/config.make