configure.ac: Respect CPPFLAGS from environment

Signed-off-by: Andreas Bombe <aeb@debian.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

diff --git a/configure.ac b/configure.ac
index 98459d4..a749794 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -48,7 +48,7 @@ AC_ARG_WITH(kernel-headers,
      esac],
     [KERNEL_HEADERS_DIR="/usr/src/kernel-headers"])
 
-CPPFLAGS="-I$KERNEL_HEADERS_DIR/include"
+CPPFLAGS="$CPPFLAGS -I$KERNEL_HEADERS_DIR/include"
 
 # Checks for header files.
 AC_CHECK_HEADERS([linux/media.h \