configure.ac: Use AC_CHECK_HEADER to detect the efi includes

configure.ac: Use AC_CHECK_HEADER to detect the efi includes

While cross-compiling, AC_CHECK_FILE will abort the configure.

The gnu-efi sources don't use relative paths and require the user to
explicitly include -I/usr/include/efi/${ARCH}. I haven't found a way to
do this with AC_CHECK_HEADER. However, since the existing test was not
testing for usability (conftest.c compilation), we don't lose much by
just not looking for efi.h and assume it exists if we can find and use
efibind.h.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>