Rename filter entry's strcpy_special() to strcpy_regex() to better

reflect its purpose.  Modify switch to support REG_EXTENDED expressions.
Use REG_EXTENDED instead of REG_BASIC everywhere.
Modify <ac/regex.h> to support <gnuregex.h> for testing purposes.
Requires: env CPPFLAGS="-DHAVE_GNUREGEX_H=1" LIBS=-lgnuregex ./configure