Change the way the pkgconfig-file install path is set for Qt5 ports.

As noticed in x11-toolkits/qt5-charts we never corrected the way in which qmake
sets the pkgconfig path for Qt5-modules. Instead there was a sed-call on the
generated Makefiles in bsd.qt.mk in the target qt-pre-install.

This diff modifies devel/qmake5 to set QMAKE_PKGCONFIG_DESTDIR to
../libdata/pkgconfig from the default pkgconfig.

As we split Qt-base into multiple ports, simply patching devel/qmake5 is not
enough, as the other base-ports will use the file from inside the tarball.
Therefore add it as an extrapatch for the Qt-base ports.
As this affects the same file extrapatch-libtool in devel/qt5 touched, rename
this one and add a hunk to do this change too.

Note: qt4 ports are unaffected, as they never were converted to used the
qt-preinstall target, and still contain the sed-call in their Makefiles.

Reviewed by: rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D8713