Rework locale feature tests

Support glibc 2.26 which removes xlocale.h. Fixes bug 788264. Thanks to
Quentin Minster for the report.

Drop support for ancient glibc 2.2.x. This allows to replace the test
program in configure.ac with a simple check for strxfrm_l.

Always use WinAPI locales on Windows.

Define macros in xsltlocale.h and rename XSLT_LOCALE_XLOCALE to
XSLT_LOCALE_POSIX.