configure.ac: Check for grep with AC_PROG_GREP

configure.ac: Check for grep with AC_PROG_GREP

Perhaps with a new version of autoconf, I began seeing:

| checking the name lister (/usr/bin/nm -B) interface... ./configure: line 6973: External.*some_variable: command not found
| BSD nm

This is because AC_PROG_NM expands to

	...
	if $GREP 'External.*some_variable' conftest.out > /dev/null; then
	    lt_cv_nm_interface="MS dumpbin"
	fi
	...

I'm not sure if it's a bug in AC_PROG_NM that it doesn't call
AC_PROG_GREP, but it's easy enough for us to do it.