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