configure: add has_or_warn

By popular request, add a function to only warn on missing stuff.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

reporter_name="Remy Bohmer"

reporter_url="linux@bohmer.net"

reporter_comment="Example for building a toolchain for m68000 bare metal targets"