Makefile: Improve compiler header dependency check

The Makefile enables CHECK_HEADER_DEPENDENCIES when the
compiler supports generating header dependencies.
Make the check use the same flags as the invocation
to avoid a false positive when user-configured compiler
flags contain incompatible options.

For example, without this patch, trying to build universal
binaries on a Mac using CFLAGS='-arch i386 -arch x86_64'
produces:

	gcc-4.2: -E, -S, -save-temps and -M options are
	not allowed with multiple -arch flags

While at it, remove "sh -c" in the command passed to $(shell);
at this point in the Makefile, SHELL has already been set to
a sensible shell and it is better not to override that.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1234,9 +1234,9 @@ COMPUTE_HEADER_DEPENDENCIES =
 USE_COMPUTED_HEADER_DEPENDENCIES =
 else
 ifndef COMPUTE_HEADER_DEPENDENCIES
-dep_check = $(shell sh -c \
-	'$(CC) -c -MF /dev/null -MMD -MP -x c /dev/null -o /dev/null 2>&1; \
-	echo $$?')
+dep_check = $(shell $(CC) $(ALL_CFLAGS) \
+	-c -MF /dev/null -MMD -MP -x c /dev/null -o /dev/null 2>&1; \
+	echo $$?)
 ifeq ($(dep_check),0)
 COMPUTE_HEADER_DEPENDENCIES=YesPlease
 endif