Better support for compiling with system libraries (#1447)

chatterino.pro

@@ -1,3 +1,16 @@
+# Exposed build flags:
+# from lib/fmt.pri
+#  - FMT_PREFIX ($$PWD by default)
+#  - FMT_SYSTEM (1 = true) (Linux only, uses pkg-config)
+# from lib/websocketpp.pri
+#  - WEBSOCKETPP_PREFIX ($$PWD by default)
+#  - WEBSOCKETPP_SYSTEM (1 = true) (unix only)
+# from lib/rapidjson.pri
+#  - RAPIDJSON_PREFIX ($$PWD by default)
+#  - RAPIDJSON_SYSTEM (1 = true) (Linux only, uses pkg-config)
+# from lib/boost.pri
+#  - BOOST_DIRECTORY (C:\local\boost\ by default) (Windows only)
+
 QT                += widgets core gui network multimedia svg concurrent
 CONFIG            += communi
 COMMUNI           += core model util
@@ -31,6 +44,9 @@ win32-msvc* {
 linux {
     LIBS += -lrt
     QMAKE_LFLAGS += -lrt
+
+    # Enable linking libraries using PKGCONFIG += libraryname
+    CONFIG += link_pkgconfig
 }
 
 macx {