Scripts: Put /usr/xpg4/bin to the beginning of PATH on Solaris.

Scripts: Put /usr/xpg4/bin to the beginning of PATH on Solaris.

This adds a configure option --enable-path-for-scripts=PREFIX
which defaults to empty except on Solaris it is /usr/xpg4/bin
to make POSIX grep and others available. The Solaris case had
been documented in INSTALL with a manual fix but it's better
to do this automatically since it is needed on most Solaris
systems anyway.

Thanks to Daniel Richard G.