pcre/2 c/should be found via pkgconfig

pcre and pcre2 can be configured to install their headers in a directory under the install prefix, which can be useful to avoid having to add `-I$prefix/include` to the compiler options.

When this is done, the build system will try to use Qt's own copy of the pcre2 libraries, or raise an error when `#include <pcre2.h>` fails.

Getting the correct compiler options via pkgconfig appears to be trivial, in qtbase/src/corelib/configure.json