allow building privoxy with a statically linked external pcre library on windows

allow building privoxy with a statically linked external pcre library on windows

see /usr/i686-w64-mingw32/sys-root/mingw/include/pcre.h line 54
  #if defined(_WIN32) && !defined(PCRE_STATIC)
  #  ifndef PCRE_EXP_DECL
  #    define PCRE_EXP_DECL  extern __declspec(dllimport)
  #  endif
If you want to statically link a program against a PCRE library in the form of
a non-dll .a file, you must define PCRE_STATIC before including pcre.h or
pcrecpp.h, otherwise the pcre_malloc() and pcre_free() exported functions will
be declared __declspec(dllimport), with unwanted results.