keymap: linux/input.h - get absolute include path from gcc

Makefile.am

@@ -546,7 +546,7 @@ udevkeymapforcereldir = $(libexecdir)/keymaps/force-release
 dist_udevkeymapforcerel_DATA = \
 	extras/keymap/force-release-maps/samsung-other
 
-extras/keymap/keys.txt: /usr/include/linux/input.h
+extras/keymap/keys.txt: $(INCLUDE_PREFIX)/linux/input.h
 	mkdir -p extras/keymap
 	$(AM_V_GEN)$(AWK) '/^#define.*KEY_[^ ]+[ \t]+[0-9]/ { if ($$2 != "KEY_MAX") { print $$2 } }' < $< | sed 's/^KEY_COFFEE$$/KEY_SCREENLOCK/' > $@
 

configure.ac

@@ -81,6 +81,9 @@ if test "x$enable_extras" = xyes; then
 			AC_MSG_ERROR([pci.ids not found, try --with-pci-ids-path=])
 		fi])
 	AC_SUBST(PCI_DATABASE)
+
+	AC_CHECK_HEADER([linux/input.h], [:], AC_MSG_ERROR([kernel headers not found]))
+	AC_SUBST([INCLUDE_PREFIX], [$(echo '#include <linux/input.h>' | eval $ac_cpp -E - | sed -n '/linux\/input.h/ {s:.*"\(.*\)/linux/input.h".*:\1:; p; q}')])
 fi
 AM_CONDITIONAL([ENABLE_EXTRAS], [test "x$enable_extras" = xyes])
 
@@ -126,6 +129,8 @@ AC_MSG_RESULT([
 	mandir:			${mandir}
 	includedir:		${includedir}
 
+	include_prefix:		${INCLUDE_PREFIX}
+
 	logging:		${enable_logging}
 	debug:			${enable_debug}
 	selinux:		${with_selinux}