libtool: Use AC_CHECK_PROG instead of AC_CHECK_TOOL to find "file"

libtool: Use AC_CHECK_PROG instead of AC_CHECK_TOOL to find "file"

This fixes a warning when cross-building:

	checking for arm-v7a-linux-gnueabihf-file... no
	checking for file... file
	configure: WARNING: using cross tools not prefixed with host triplet

file isn't platform specific and not usually installed with a host
triplet. So use AC_CHECK_PROG which differs from AC_CHECK_TOOL by not
expecting such a host triplet prefix.

* m4/libtool.m4 (_LT_DECL_FILECMD): Change AC_CHECK_TOOL to AC_CHECK_PROG.