Update buildkey for GCC 5

[taken from RedHat Qt3 patches]
(cherry picked from commit fa440833f9)

configure

@@ -2810,7 +2810,7 @@ case "$COMPILER" in
 g++*)
     # GNU C++
     QMAKE_CONF_COMPILER=`grep "QMAKE_CXX[^_A-Z0-9a-z]" $QMAKESPEC/qmake.conf | sed "s,.* *= *\(.*\)$,\1,"`
-    COMPILER_VERSION=`${QMAKE_CONF_COMPILER} --version 2>/dev/null | sed 's,^[^0-9]*,,g'`
+    COMPILER_VERSION=`${QMAKE_CONF_COMPILER} -dumpversion 2>/dev/null | sed 's,^[^0-9]*,,g'`
     case "$COMPILER_VERSION" in
     2.95.*)
 	COMPILER_VERSION="2.95.*"
@@ -2818,7 +2818,7 @@ g++*)
     3.*)
 	COMPILER_VERSION="3.*"
 	;;
-    4.*)
+    5.*|4.*)
 	COMPILER_VERSION="4.*"
 	;;
     *)