Add --with-static-standard-libraries to the top level

Add --with-static-standard-libraries to the top level

gdb should normally not be linked with -static-libstdc++.  Currently
this has not caused problems, but it's incompatible with catching an
exception thrown from a shared library -- and a subsequent patch
changes gdb to do just this.

This patch adds a new --with-static-standard-libraries flag to the
top-level configure.  It defaults to "auto", which means enabled if
gcc is being built, and disabled otherwise.

ChangeLog
2019-08-19  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* configure.ac: Add --with-static-standard-libraries.