suppress compiler output in compilerSupportsFlag()

that also covers linkerSupportsFlag(), which did it explicitly so far.

Task-number: QTBUG-45010
Change-Id: I2eb0bd5282fd2f24c9ab8041c3536a6115caa765
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

configure

@@ -356,7 +356,7 @@ compilerSupportsFlag()
     cat >conftest.cpp <<EOF
 int main() { return 0; }
 EOF
-    "$@" -o conftest-out.o conftest.cpp
+    "$@" -o conftest-out.o conftest.cpp >/dev/null 2>&1
     ret=$?
     rm -f conftest.cpp conftest-out.o
     return $ret
@@ -372,7 +372,7 @@ linkerSupportsFlag()
 	safe_flag=`shellEscape "$flag"`
 	lflags=$lflags,$safe_flag
     done
-    compilerSupportsFlag $compiler $lflags >/dev/null 2>&1
+    compilerSupportsFlag $compiler $lflags
 }
 
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