[PATCH resend] Makefile: Use computed header dependencies if the compiler supports it

[Fredrik Kuivinen <frekui@gmail.com>]

Previously you had to manually define COMPUTE_HEADER_DEPENDENCIES to
enable this feature. It seemed a bit sad that such a useful feature
had to be enabled manually.

Signed-off-by: Fredrik Kuivinen <frekui@gmail.com>
---

This is a resend, it has been rebased on top of master but otherwise
the same patch was sent 2011-06-11. Jonathan Nieder has been added to
the Cc list as he implemented the computed header dependencies
feature.

 Makefile |   13 +++++++++----
 1 files changed, 9 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index 8dd782f..c289074 100644
--- a/Makefile
+++ b/Makefile
@@ -250,10 +250,6 @@ all::
 #   DEFAULT_EDITOR='$GIT_FALLBACK_EDITOR',
 #   DEFAULT_EDITOR='"C:\Program Files\Vim\gvim.exe" --nofork'
 #
-# Define COMPUTE_HEADER_DEPENDENCIES if your compiler supports the -MMD option
-# and you want to avoid rebuilding objects when an unrelated header file
-# changes.
-#
 # Define CHECK_HEADER_DEPENDENCIES to check for problems in the hard-coded
 # dependency rules.
 #
@@ -1236,6 +1232,15 @@ endif
 ifdef CHECK_HEADER_DEPENDENCIES
 COMPUTE_HEADER_DEPENDENCIES =
 USE_COMPUTED_HEADER_DEPENDENCIES =
+else
+dep_check = $(shell sh -c \
+	': > ++empty.c; \
+	$(CC) -c -MF /dev/null -MMD -MP ++empty.c -o /dev/null 2>&1; \
+	echo $$?; \
+	$(RM) ++empty.c')
+ifeq ($(dep_check),0)
+COMPUTE_HEADER_DEPENDENCIES=YesPlease
+endif
 endif
 
 ifdef COMPUTE_HEADER_DEPENDENCIES
-- 
1.7.5.3.368.g8b1b7.dirty