configure, gbuild: remove Cygwin paths from make environment

All environment variables contain native Win32 paths.

This allows building with Win32 make and Cygwin sh.exe,
but Cygwin make cannot build any more with the Win32 entries in PATH.

Requires adding "bash" detection to configure since /usr/bin/env
can't be invoked by Win32 make and it's possible to install cygwin
somewhere else than C:/cygwin.

Change-Id: Ia8f4ee3c91222e8f5919cfbfc452f12ffbfadd40
Reviewed-on: https://gerrit.libreoffice.org/9698
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>