BUILD: makefile: support PCRE[2]_CFLAGS and PCRE[2]_LDFLAGS

BUILD: makefile: support PCRE[2]_CFLAGS and PCRE[2]_LDFLAGS

The CFLAGS and LDFLAGS appended by USE_PCRE/USE_PCRE2 can now be
overridden using PCRE_CFLAGS/PCRE2_CFLAGS and PCRE_LDFLAGS/PCRE2_LDFLAGS.

Its worth noting that PCRE2_LDFLAGS did already exist and was preset from
the pkgconfig output then complemented with -lpcre2-posix, and only then
the -L and optional -Wl,-Bstatic were appended when adding them to the
resulting global LDFLAGS. A search on the net did not reveal any use of
PCRE2_LDFLAGS in any public build scripts, and for consistency sake it's
important to make sure that we can now finally override the -L settings
like we're able to do with every other build option. Thus the meaning of
this variable changed to include all the related ldflags (-L and -Wl).

These flags are now automatically collected into OPTIONS_*.