Build system: handle source dependencies

Makefile

@@ -180,6 +180,9 @@ CLEAN_PATH_TMPL += clean-sdkdir-tmpl
 # Enable secondary expansion.
 .SECONDEXPANSION:
 
+# Include source dependency files.
+-include $(call get-deps,$(objs-all))
+
 # Rule to archive prerequisite objects into a library.
 $(lib-all): | $$(@D)
 	$(call info-cmd,LIB)

README.md

@@ -228,6 +228,11 @@ source tree is replicated in there, to avoid conflicts between files with the sa
 you wanted to compile the file `src/foo/bar.c` for an eVic VTC Mini, debug flavor, you could
 issue `make obj/dbg/evic/src/foo/bar.o`.
 
+#### Source dependency files
+
+Source dependency files live in the same directory as object files. They have a `.d` extension
+and are generated automatically when objects are built.
+
 #### SDK builds
 
 SDK output files live in the `lib/FLAVOR/DEVICE` directory. Debug builds enable debug info

make/Base.mk

@@ -105,6 +105,9 @@ CLEAN_PATH_TMPL += clean-bindir-tmpl
 # Enable secondary expansion.
 .SECONDEXPANSION:
 
+# Include source dependency files.
+-include $(call get-deps,$(objs-all))
+
 # Set BUILD_* for all our targets.
 $(call build-vars-rules,elf-tmpl)
 $(call build-vars-rules,bin-dec-tmpl)

make/Common.mk

@@ -144,10 +144,12 @@ get-incflags = $(foreach d,$(call fixpath-cc,$1),-I$d)
 # CXXFLAGS: C++ compiler flags.
 # ASFLAGS: assembler flags.
 # Note that those are NOT inclusive of CPU flags.
+# The -MMD flag generates source dependency files. Those files
+# are never specified as targets to avoid a remake on include.
 __CC_FLAGS := \
 	$(call get-incflags,$(INCDIRS)) \
 	-Os -fdata-sections -ffunction-sections \
-	$(if $(CC_IS_CLANG),-nostdinc)
+	$(if $(CC_IS_CLANG),-nostdinc) -MMD
 ifndef EVICSDK_FPU_DISABLE
 	__CC_FLAGS += -DEVICSDK_FPU_SUPPORT
 	ASFLAGS += -DEVICSDK_FPU_SUPPORT
@@ -253,6 +255,9 @@ clean-devfla-tmpl = clean$(if $1,-$1)$(if $2,-$2)
 # Gets the paths to all possible sources for the given objects.
 # Argument 1: object list.
 get-srcs = $(foreach e,c cpp s,$(patsubst %.o,%.$e,$1))
+# Gets the paths to all source dependency files for the given objects.
+# Argument 1: object list.
+get-deps = $(addsuffix .d,$(basename $1))
 
 # Gets object and source paths to cache for fixpath.
 # Argument 1: object list.