With CONFIG=…, the default values of CONFIG are cleared, with qt4 this
doesn't seem to be a problem, but with qt5 the build fails.
CONFIG=staticlib in openpgpsdk.pro was added in 3caeb0ab7f
for OS X, but is not necessary on linux
The icons and desktop file in the data directory are shared between
linux distributions so they shouldn't be in the build_scripts directory.
The redhat build now makes use of "make install" and relocatable plugins dir.