Build fixes for recent libc versions (post glibc 2.19)

Build fixes for recent libc versions (post glibc 2.19)

This fixes two 'undefined reference' errors when building with a recent
glibc.  One in effi.c for 'dlsym', and the other in emacs.c for
'memory_warnings'.

	* configure.ac: Fix mmap detection.
	Include -ldl in FFI_LIBS.
	Ensure that vm-limit.o is added to the objects when using the
	system malloc.
	Default to using the system malloc.

	* m4/sxe-emodffi.m4 (SXE_CHECK_FFI): Include -ldl in FFI_LIBS.

Signed-off-by: Steve Youngs <steve@sxemacs.org>