Fix QMAKE_CXX/CROSS_COMPILE verification with ccache

The use of ccache leads to QMAKE_CXX definitions of the form:

    QMAKE_CXX = $${CCACHE} $${CROSS_COMPILE}g++

The previous test required QMAKE_CXX to be a single valid (absolute or
QMAKE_PATH_ENV-relative) path to an existing file, which was not
compatible with definitions of QMAKE_CXX like the one above.

Fix this by using only the first value in QMAKE_CXX, which usually
points to the compiler executable, or to the ccache executable in the
above case.

Task-number: QTBUG-47951
Change-Id: Iade3136f03493593b067fb7742fb997f92377425
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>

mkspecs/features/device_config.prf

@@ -19,10 +19,15 @@ defineTest(deviceSanityCheckCompiler) {
     else: \
         sfx =
 
+    # Build the compiler filename using the first value in QMAKE_CXX in order to
+    # support tools like ccache, which give QMAKE_CXX values of the form:
+    #     ccache <path_to_compiler>
+    compiler = $$first(QMAKE_CXX)$$sfx
+
     # Check if the binary exists with an absolute path. Do this check
     # before the CROSS_COMPILE empty check below to allow the mkspec
     # to derive the compiler path from other device options.
-    exists($$QMAKE_CXX$$sfx):return()
+    exists($$compiler):return()
 
     # Check for possible reasons of failure
     # check if CROSS_COMPILE device-option is set
@@ -31,7 +36,7 @@ defineTest(deviceSanityCheckCompiler) {
     # Check if QMAKE_CXX points to an executable.
     ensurePathEnv()
     for (dir, QMAKE_PATH_ENV) {
-        exists($$dir/$${QMAKE_CXX}$$sfx): \
+        exists($$dir/$${compiler}): \
             return()
     }