Add cross compiling pkg-config disabled warning
Add cross compiling pkg-config disabled warning

We have historically silently disabled pkg-config when cross compiling in
order to avoid host pkg-config contamination of the target build.
pkg-config files are present on many embedded targets, and make the correct
detection/configuration of dependencies far more convenient for the
developer. It is therefor valuable to advertise that pkg-config
functionality is available but disabled barring direct involvement on their
parts, rather than silently disabling it.

Reviewed-by: Thiago Macieira <0ed1648d92ca0373297a182b1dba1b974ad10d88@nokia.com>

igor-sfdc/qt-wk,radekp/qt,radekp/qt,radekp/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,radekp/qt,radekp/qt,radekp/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,radekp/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,radekp/qt,igor-sfdc/qt-wk