Fix build from gitversion.h-less source trees

Having proper gitversion.h helps tracking software versions
in the wild. This commit makes source trees that lack the
gitversion.h file for one reason or another buildable.

main.c

@@ -17,7 +17,7 @@ int main(void)
   printf(
     "JACK Plugin Launcher, version " JPL_VERSION "\n"
     "Copyleft JACK Plugin Authors\n"
-#if (HAVE_GITVERSION_H)
+#if defined(GIT_VERSION)
     "built from " GIT_VERSION "\n"
 #endif
     "built on " BUILD_TIMESTAMP"\n"

wscript

@@ -46,7 +46,8 @@ def git_ver(self):
         self.ver = "unknown"
 
     fi = open(header, 'w')
-    fi.write('#define GIT_VERSION "%s"\n' % self.ver)
+    if self.ver != "unknown":
+        fi.write('#define GIT_VERSION "%s"\n' % self.ver)
     fi.close()
 
 def display_msg(conf, msg="", status = None, color = None):