Add cross-compilation support on Linux

Add cross-compilation support on Linux

This adds three new command-line options to the configure script:

--cross_compile=<prefix> (default "")
--cc=<compiler>          (default cc)
--cflags=<compile flags> (default -g -O2 -pipe)

These get propagated to the Makefiles in the subdirectories.  The
cross-compile prefix is prepended to the CC value, so for example
if you do "./configure --cross_compile=powerpc64le-linux-" then
everything gets compiled and linked using powerpc64le-linux-cc.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>