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
@@ -1,5 +1,6 @@
 # Files I don't care to see in git-status/commit
 cgit
 cgit.conf
 VERSION
 *.o
+*.d
diff --git a/Makefile b/Makefile
index d1950cb..faf7b98 100644
--- a/Makefile
+++ b/Makefile
@@ -10,12 +10,19 @@ GIT_URL = http://www.kernel.org/pub/software/scm/git/git-$(GIT_VER).tar.bz2
 #
 # Let the user override the above settings.
 #
 -include cgit.conf
 
 
+#
+# 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
 OBJECTS =
 OBJECTS += cache.o
 OBJECTS += cgit.o
 OBJECTS += cmd.o
 OBJECTS += html.o
@@ -58,12 +65,14 @@ CFLAGS += -DCGIT_CACHE_ROOT='"$(CACHE_ROOT)"'
 
 cgit: $(OBJECTS)
         $(CC) $(CFLAGS) -o cgit $(OBJECTS) $(EXTLIBS)
 
 $(OBJECTS): git/xdiff/lib.a git/libgit.a VERSION
 
+-include $(OBJECTS:.o=.d)
+
 git/xdiff/lib.a: | git
 
 git/libgit.a: | git
 
 git:
         cd git && $(MAKE) xdiff/lib.a
@@ -81,13 +90,13 @@ install: all
 uninstall:
         rm -f $(CGIT_SCRIPT_PATH)/$(CGIT_SCRIPT_NAME)
         rm -f $(CGIT_SCRIPT_PATH)/cgit.css
         rm -f $(CGIT_SCRIPT_PATH)/cgit.png
 
 clean:
-        rm -f cgit VERSION *.o
+        rm -f cgit VERSION *.o *.d
         cd git && $(MAKE) clean
 
 distclean: clean
         git clean -d -x
         cd git && git clean -d -x