Add -Wextra to the default compiler settings for GCC.

(I removed this from my on-commit autobuilder custom flags.)
Note that it doesn't have -Werror.


git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@1302 e5f2f494-b856-4b98-b285-d166d9295462

configure.ac

@@ -23,7 +23,7 @@ AM_PATH_PYTHON([3.1])
 
 # default compiler warning settings
 if test "X$GCC" = "Xyes"; then
-CXXFLAGS="$CXXFLAGS -g -Wall -Wwrite-strings -Woverloaded-virtual -Wno-sign-compare"
+CXXFLAGS="$CXXFLAGS -g -Wall -Wextra -Wwrite-strings -Woverloaded-virtual -Wno-sign-compare"
 fi
 
 # produce PIC unless we disable shared libraries. need this for python bindings.