Replace qmake variable BLACK_CONFIG with a system based on json files.

The config is loaded from json files by qmake, and we define the qmake
function `swiftConfig` for checking whether a feature is enabled.
This function can be directly used in `buildconfig_gen.cpp.in`,
so the trick with C++ comment tokens in variables is not needed.

src/blackinput/blackinput.pro

@@ -7,7 +7,7 @@ TARGET = blackinput
 TEMPLATE = lib
 CONFIG   += blackmisc
 
-contains(BLACK_CONFIG, Static) {
+swiftConfig(static) {
     CONFIG += staticlib
 }