INSTALL.md: Add note about EXTRA_CFLAGS.

Add a note about the use of EXTRA_CFLAGS to provide custom CFLAGS for
the build of the Linux kernel module.

This addition is technically in the "configuring the sources" section
of the document.  However, it's the spot where custom CFLAGS is
discussed already, so that seemed like the best place to put it.
Alternatively, it could go in the "Building the Sources" section
instead.

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>

INSTALL.md

@@ -204,6 +204,12 @@ default CFLAGS plus "-mssse3", you might run configure as follows:
 
       `% ./configure CFLAGS="-g -O2 -mssse3"`
 
+Note that these CFLAGS are not applied when building the Linux
+kernel module.  Custom CFLAGS for the kernel module are supplied
+using the EXTRA_CFLAGS variable when running make.  So, for example:
+
+      `% make EXTRA_CFLAGS="-Wno-error=date-time"
+
 To build the Linux kernel module, so that you can run the
 kernel-based switch, pass the location of the kernel build
 directory on --with-linux.  For example, to build for a running