configure: support CPPFLAGS

Useful for accessing APIs that are still brewing, e.g:

  CROSS_PREFIX=aarch64-linux-gnu- \
    CPPFLAGS=-I/home/alex/lsrc/qemu/risu.git/sve-headers/include \
    ../configure

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20171107150558.22131-7-alex.bennee@linaro.org
[PMM: tweaked text in README for typos etc]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

2 files changed, 7 insertions, 0 deletions

diff --git a/README b/README
index 312e9cd..e90d33c 100644
--- a/README
+++ b/README
@@ -26,6 +26,12 @@ Most useful is
    need this if you're not building on the target system
    (Example: CROSS_PREFIX=arm-linux-gnueabihf- )
 
+Another useful flag is
+ CPPFLAGS= which specifies pre-processor flags, usually -I statements
+   for specifying extra include paths. Use this if you need something
+   from new kernel headers not installed on your system, for instance.
+   (Example: CPPFLAGS=-I/path/to/new-kernel-headers/include)
+
 Passing --static will build a statically linked binary which is useful
 if you don't want to mess around with a chroot to run the binary.
 
diff --git a/configure b/configure
index c622a5e..65e1819 100755
--- a/configure
+++ b/configure
@@ -111,6 +111,7 @@ generate_makefilein() {
     echo "# Makefile.in - generated by the 'configure' script" > $m
     echo "ARCH:=${ARCH}" >> $m
     echo "CC:=${CC}" >> $m
+    echo "CPPFLAGS:=${CPPFLAGS}" >> $m
     echo "LDFLAGS:=${LDFLAGS}" >> $m
     echo "AS:=${AS}" >> $m
     echo "OBJCOPY:=${OBJCOPY}" >> $m