Find PCRE2 via pkgconfig

This avoids issues when using dependencies installed in a custom or
dedicated $prefix, when PCRE2 is configured to install its headers
in their own directory under instead of directly in $prefix/include.

Task-number: QTBUG-66355
Change-Id: Ia99464820600282d61237e5bcff38101d2c9684b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@qt.io>

src/corelib/configure.json

@@ -176,6 +176,7 @@
                 ]
             },
             "sources": [
+                { "type": "pkgConfig", "args": "libpcre2-16" },
                 "-lpcre2-16"
             ]
         },