Makefile: autobuild dependency rules

This uses gcc to generate dependency rules for each `.o` file, based on the
corresponding `.c` file, into a new set of `.d` files (which are also defined
to depend on the same set of source files as their `.o` files).

Result:
* all objectfile dependencies are correctly calculated
* only the necessary dependencies are recalculated when a sourcefile is updated

Inspiration for the build rules:
* http://www.gnu.org/software/make/manual/make.html#Automatic-Prerequisites
* http://make.paulandlesley.org/autodep.html

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

2 files changed, 11 insertions, 1 deletions

diff --git a/.gitignore b/.gitignore
index aa36ff7..1e016e5 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5 +5,2 @@ VERSION
 *.o
+*.d
diff --git a/Makefile b/Makefile
index d1950cb..faf7b98 100644
--- a/Makefile
+++ b/Makefile
@@ -15,2 +15,9 @@ GIT_URL = http://www.kernel.org/pub/software/scm/git/git-$(GIT_VER).tar.bz2
 
+#
+# Define a pattern rule for automatic dependency building
+#
+%.d: %.c
+        $(CC) $(CFLAGS) -MM $< | sed -e 's/\($*\)\.o:/\1.o $@:/g' >$@
+
+
 EXTLIBS = git/libgit.a git/xdiff/lib.a -lz -lcrypto
@@ -63,2 +70,4 @@ $(OBJECTS): git/xdiff/lib.a git/libgit.a VERSION
 
+-include $(OBJECTS:.o=.d)
+
 git/xdiff/lib.a: | git
@@ -86,3 +95,3 @@ uninstall:
 clean:
-        rm -f cgit VERSION *.o
+        rm -f cgit VERSION *.o *.d
         cd git && $(MAKE) clean