Better support for compiling with system libraries (#1447)

lib/rapidjson.pri

@@ -1,2 +1,15 @@
 # rapidjson
-INCLUDEPATH += $$PWD/rapidjson/include/
+# Chatterino2 is tested with RapidJSON v1.1.0
+#  - RAPIDJSON_PREFIX ($$PWD by default)
+#  - RAPIDJSON_SYSTEM (1 = true) (Linux only, uses pkg-config)
+
+!defined(RAPIDJSON_PREFIX) {
+    RAPIDJSON_PREFIX = $$PWD
+}
+
+linux:equals(RAPIDJSON_SYSTEM, "1") {
+    message("Building with system RapidJSON")
+    PKGCONFIG += RapidJSON
+} else {
+    INCLUDEPATH += $$RAPIDJSON_PREFIX/rapidjson/include/
+}