Merge branch 'ab/pcre-v2'

Merge branch 'ab/pcre-v2'

Git can be built to use either v1 or v2 of the PCRE library, and so
far, the build-time configuration USE_LIBPCRE=YesPlease instructed
the build procedure to use v1, but now it means v2.  USE_LIBPCRE1
and USE_LIBPCRE2 can be used to explicitly choose which version to
use, as before.

* ab/pcre-v2:
  Makefile: make USE_LIBPCRE=YesPlease mean v2, not v1
  configure: detect redundant --with-libpcre & --with-libpcre1
  configure: fix a regression in PCRE v1 detection