Add a Makefile to figure out the compiler flags for kernel builds

Author: geofft

kernel-cflags-finder/.gitignore

@@ -0,0 +1,13 @@
+.built-in.o.cmd
+.dummy.ko.cmd
+.dummy.mod.o.cmd
+.dummy.o.cmd
+.tmp_versions/
+Module.symvers
+built-in.o
+dummy.c
+dummy.ko
+dummy.mod.c
+dummy.mod.o
+dummy.o
+modules.order

kernel-cflags-finder/Makefile

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+ifneq ($(KERNELRELEASE),)
+obj-m += dummy.o
+clean-files := dummy.c
+
+$(M)/dummy.c:
+	@echo $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(__c_flags) $(modkern_cflags)
+	@touch $@
+else
+KDIR ?= /lib/modules/$(shell uname -r)/build
+CLANG ?= clang
+all:
+	$(MAKE) -C $(KDIR) M=$(CURDIR) CC=$(CLANG)
+clean:
+	$(MAKE) -C $(KDIR) M=$(CURDIR) clean
+endif

kernel-cflags-finder/README.md

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+Prints the compiler arguments needed to run Clang against the kernel
+headers. We use this for running libclang from bindgen.
+
+Normal usage: run `make -s` and look at stdout (errors go to stderr). If
+you need to build for a specific set of kernel headers, add
+something like `KDIR=/lib/modules/3.16.0-4-amd64/build`. If your clang
+command is not `clang`, add something like `CLANG=clang-3.8`.
+
+This generates a non-functional kernel module in the process. To clean
+up, `make clean`.