[PATCH]: Don't build testsuite during normal build on Cygwin

[Benjamin Kosnik]

It would be nice to do this in configure.target instead of configure.in.

Since things go

configure.target
GLIBCPP_CONFIGURE_TESTSUITE
configure.in hack as proposed

Seems odd. If it can't be done in configure.target, then maybe in
acinclude.m4 in, say:

  # Don't do ABI checking unless native or cygwin
  AM_CONDITIONAL(GLIBCPP_BUILD_ABI_CHECK,
                 test x"$build" = x"$host" && test -z "$with_cross_host"
                  && test x"$target" != cygwin)

or whatever

-benjamin