windows build: msvc vs mingw - missing __FUNCSIG__ macro

[pvanek]

currently Slaq cannot be built with Win/MinGW toolchain. This simple patch fixes it for MinGW:

diff --git a/slaq.pro b/slaq.pro
index 1a0551e..23190e8 100644
--- a/slaq.pro
+++ b/slaq.pro
@@ -28,7 +28,9 @@ CONFIG(debug, debug|release) {
 }

 DEFINES += SLAQ_VERSION=\\\"$$VERSION\\\"
-windows: DEFINES += __PRETTY_FUNCTION__=__FUNCSIG__
+win32-msvc* {
+    windows: DEFINES += __PRETTY_FUNCTION__=__FUNCSIG__
+}
 # Translations
 TRANSLATIONS += translations/slaq-fi.ts