pay attention to the MAKE env variable

... to avoid that inherited MAKEFLAGS turn out to be incompatible with
the make we choose.

Task-number: QTBUG-39527
Change-Id: I47d4cec58b0643cc5d97868e70b24f7f37e964bb
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>

mkspecs/features/configure.prf

-equals(MAKEFILE_GENERATOR, UNIX) {
+QMAKE_MAKE = $$(MAKE)
+!isEmpty(QMAKE_MAKE) {
+    # We were called recursively. Use the right make, as MAKEFLAGS may be set as well.
+} else:equals(MAKEFILE_GENERATOR, UNIX) {
     QMAKE_MAKE = make
 } else:equals(MAKEFILE_GENERATOR, MINGW) {
     !equals(QMAKE_HOST.os, Windows): \