Remove undefined SFL_CXX_WITH_DEBUG macro

It was originally defined in the toplevel m4/sfl-internal.m4 file and
later inlined in the configure.ac. While the definition removed, the
reference was not and causes the configure script to print an error
message.

sflphone-common/configure.ac

@@ -90,7 +90,6 @@ AC_SUBST(PKGADD_VENDOR)
 
 	dnl Check for programs
 	AC_PROG_CC
-	SFL_CXX_WITH_DEBUG
 	AC_PROG_CXX
 	AC_PROG_CPP
 	AC_PROG_INSTALL