refs #461 Only complain about missing externals when qmake is really generating build files.

This is so we can use the fromfile function to extract variables from pro files.

.qmake.conf

@@ -2,3 +2,4 @@ SourceRoot=$$PWD
 BuildRoot=$$shadowed($$SourceRoot)
 DestRootDebug=$$BuildRoot/out/debug
 DestRootRelease=$$BuildRoot/out/release
+CONFIG+=in_full_qmake

mkspecs/features/common_pre.prf

@@ -107,7 +107,7 @@ isEmpty(EXTERNALSROOT) {
     EXTERNALSROOT = $$SourceRoot/externals
 }
 
-!exists("$$EXTERNALSROOT/common/include") {
+!exists("$$EXTERNALSROOT/common/include"):in_full_qmake {
     error("Could not find externals in $$EXTERNALSROOT. Please install it!")
 }