configure: don't use -lintl when there is no gettext support

configure: don't use -lintl when there is no gettext support

The current configure script uses -lintl if gettext is not found in the C
library, but does so before checking if there is libintl.h available in
the first place, in which case we would later define NO_GETTEXT.

Instead, check for the existence of libintl.h first. Only when libintl.h
exists and libintl is not in libc, ask for -lintl.

Signed-off-by: John Szakmeister <john@szakmeister.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>