Prevent git run at every ${CC} call

Index: src/kristall.pro
--- src/kristall.pro.orig
+++ src/kristall.pro
@@ -9,7 +9,7 @@ greaterThan(QT_MAJOR_VERSION, 4): QT += widgets networ
 # deprecated API in order to know how to port your code away from it.
 DEFINES += QT_DEPRECATED_WARNINGS
 
-DEFINES += KRISTALL_VERSION="\"$(shell cd $$PWD; git describe --tags)\""
+DEFINES += KRISTALL_VERSION="OpenBSD"
 
 # You can also make your code fail to compile if it uses deprecated APIs.
 # In order to do so, uncomment the following line.