[FIX] (import_Seitosh): reset variable FC if value is default

Check whether FC is set from the value of implicit variables (origin is
default). If yes, replace value of variable. If not, FC most likely is set by
intention of the user. Do not alter the value in this case.

contrib/Makefile_var

@@ -32,7 +32,7 @@ TF_WWWBASEDIR=$(CONTRIBDIR)/doc
 TF_BROWSER=firefox
 
 # set Fortran compiler if not yet set in environment
-ifndef FC
+ifeq "$(origin FC)" "default"
 FC=/usr/bin/g77-3.3
 endif