gnatcoll.gpr: rename external GNATCOLL_BUILD into GNATCOLL_BUILD_MODE

To avoid conflict with installed gnatcoll.gpr where GNATCOLL_BUILD means
library type.

Part of S709-037.

Change-Id: I0f2d1fe21161ef9b2ba821234eb4bc22c4542fe9

Author: Fabien-Chouteau

gnatcoll.gpr

@@ -34,7 +34,7 @@ project GnatColl is
 
    type Build_Type is ("DEBUG", "PROD");
    Build : Build_Type :=
-      External ("GNATCOLL_BUILD", External ("BUILD", "DEBUG"));
+      External ("GNATCOLL_BUILD_MODE", External ("BUILD", "DEBUG"));
 
    type Library_Type_Type is ("relocatable", "static", "static-pic");
    Library_Type : Library_Type_Type := External ("LIBRARY_TYPE", "static");