CONFIG shouldn't be reset at qmake call, use CONFIG+=… instead

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

openpgpsdk/src/openpgpsdk.pro

@@ -1,10 +1,10 @@
 !include("../../retroshare.pri"): error("Could not include file ../../retroshare.pri")
 
 TEMPLATE = lib
-win32 {
-	CONFIG += staticlib
-} else {
+macx {
 	CONFIG = staticlib 
+} else {
+	CONFIG += staticlib
 }
 
 DEFINES *= OPENSSL_NO_IDEA