Fixed|libdeng2: Use the correct build number (instead of today's)

Due to use of a missing preprocessor definition, libdeng2's Version
class always ended up using today's build number instead of the
build's actual number. Consequently, libdeng2 was unable to correctly
detect up/downgrades.

Fixed the preprocessor definition in libdeng2.pro.

doomsday/libdeng2/libdeng2.pro

@@ -26,6 +26,10 @@ DEFINES += LIBDENG2_VERSION=\\\"$${LIBDENG2_MAJOR_VERSION}.$${LIBDENG2_MINOR_VER
 
 DEFINES += __DENG2__
 
+!isEmpty(DENG_BUILD) {
+    DEFINES += LIBDENG2_BUILD_TEXT=\\\"$$DENG_BUILD\\\"
+}
+
 # Using Qt.
 QT += core network
 

doomsday/libdeng2/src/version.cpp

@@ -31,8 +31,8 @@ Version::Version() : build(Time().asBuildNumber())
     minor = LIBDENG2_MINOR_VERSION;
     patch = LIBDENG2_PATCHLEVEL;
 
-#ifdef DOOMSDAY_BUILD_TEXT
-    build = String(DOOMSDAY_BUILD_TEXT).toInt();
+#ifdef LIBDENG2_BUILD_TEXT
+    build = String(LIBDENG2_BUILD_TEXT).toInt();
 #endif
 
     label = LIBDENG2_RELEASE_LABEL;