build: Use pkg-config in BITCOIN_QT_CONFIGURE for all hosts

This change adds to the BITCOIN_QT_CONFIGURE script ability to use
pkg-config for MinGW. All of the non-pkg-config paths are removed as
needless.
If depends is built with DEBUG=1 the configure script fails to pickup
Qt:
- for macOS host (similar, but not the same as issue 16391)
- for Windows host (regression)

depends/config.site.in

@@ -42,15 +42,6 @@ if test x@host_os@ = xdarwin; then
   PORT=no
 fi
 
-if test x@host_os@ = xmingw32; then
-  if test -z $with_qt_incdir; then
-    with_qt_incdir=$depends_prefix/include
-  fi
-  if test -z $with_qt_libdir; then
-    with_qt_libdir=$depends_prefix/lib
-  fi
-fi
-
 PATH=$depends_prefix/native/bin:$PATH
 PKG_CONFIG="`which pkg-config` --static"