Look for the NINJA_PATH env var and prefer that over the bootstrapped ninja

The snapshot ninja may cause issues in some environments, where a different
ninja version may work. If the user has NINJA_PATH set in their environment,
prefer that over bootstrapping our own.

Change-Id: I4c6d2f479aeae1f5137705ed2deb995b59ea287a
Reviewed-by: Andras Becsi <andras.becsi@digia.com>

tools/qmake/mkspecs/features/functions.prf

@@ -105,10 +105,15 @@ defineReplace(which) {
 }
 
 defineReplace(findOrBuildNinja) {
-    out = $$absolute_path("$${getChromiumSrcDir()}/../ninja/ninja", "$$QTWEBENGINE_ROOT")
-    win32: out = $$system_path($${out}.exe)
+    # If NINJA_PATH env var is set, prefer that.
+    # Fallback to locating our own bootstrapped ninja.
+    out = $(NINJA_PATH)
+    !exists($$out) {
+      out = $$absolute_path("$${getChromiumSrcDir()}/../ninja/ninja", "$$QTWEBENGINE_ROOT")
+      win32: out = $$system_path($${out}.exe)
 
-    # If we did not find ninja, then we bootstrap it.
-    !exists($$out): system("python $$dirname(out)/bootstrap.py")
+      # If we did not find ninja, then we bootstrap it.
+      !exists($$out): system("python $$dirname(out)/bootstrap.py")
+    }
     return($$out)
 }