buildsys: allow the toolchain to search its dirs for kernel headers

Setting KERNEL_HEADERS up to the exact path all the time is a pita,
especially when the toolchain is often times already configured to
do the right thing.  So if the user has set that to "", don't force
any specific paths.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

extra/Configs/Config.in.arch

@@ -192,7 +192,7 @@ config UCLIBC_HAS_LONG_DOUBLE_MATH
 
 config KERNEL_HEADERS
 	string "Linux kernel header location"
-	default "/usr/include"
+	default ""
 	help
 	  The kernel source you use to compile with should be the same
 	  as the Linux kernel you run your apps on.  uClibc doesn't even
@@ -203,6 +203,8 @@ config KERNEL_HEADERS
 	  but then run on Linux 2.0.x, lchown will be compiled into uClibc,
 	  but won't work at all.  You have been warned.
 
+	  If you don't set this, we'll assume the toolchain can find them.
+
 config UCLIBC_UCLINUX_BROKEN_MUNMAP
 	bool
 	depends on !ARCH_USE_MMU

extra/scripts/gen_bits_syscall_h.sh

@@ -15,7 +15,12 @@
 #
 # Warning!!! This does _no_ error checking!!!
 
-INCLUDE_OPTS="-nostdinc -I${KERNEL_HEADERS}"
+if [ "${KERNEL_HEADERS:-/}" != "/" ] ; then
+  INCLUDE_OPTS="-nostdinc -I${KERNEL_HEADERS}"
+else
+  # Let the toolchain use its configure paths.
+  INCLUDE_OPTS=
+fi
 
 case $CC in
 *icc*) CC_SYSNUM_ARGS="-dM" ;;