don't clear config.log unless re-checking

it is counterproductive to clear the log when cached test results are
used, as that makes it hard to determine how they came to be.

-recheck isn't as clear-cut as -recheck-all, as only part of the results
is discarded, and we can't reasonably discard only part of the log. i
opted for clearing the log entirely, as having both the old and new
results in the log would be probably quite confusing.

Change-Id: Ibb391f2ba2ea86d73c23365d46cc66ed8a2158d6
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

mkspecs/features/configure.prf

@@ -4,6 +4,8 @@ cache()
 
 load(configure_base)
 
+recheck: write_file($$QMAKE_CONFIG_LOG, "")
+
 isEmpty(QMAKE_CONFIG_TESTS_DIR): QMAKE_CONFIG_TESTS_DIR = $$_PRO_FILE_PWD_/config.tests
 
 # Try to build the test project in $$QMAKE_CONFIG_TESTS_DIR/$$1

mkspecs/features/configure_base.prf

@@ -44,4 +44,3 @@ defineTest(qtRunLoggedCommand) {
 }
 
 QMAKE_CONFIG_LOG = $$dirname(_QMAKE_CACHE_)/config.log
-write_file($$QMAKE_CONFIG_LOG, "")

mkspecs/features/qt_configure.prf

@@ -1729,6 +1729,8 @@ equals(QMAKE_CONFIG_CACHE_USE, none) {
         "cache.xplatform = $$[QMAKE_XSPEC]"
     write_file($$QMAKE_CONFIG_CACHE, cont)
 }
+!equals(QMAKE_CONFIG_CACHE_USE, all): \
+    write_file($$QMAKE_CONFIG_LOG, "")
 
 for (currentConfig, allConfigs) {
     qtConfSetModuleName()