Cross compilation issue (#31)

The error that happen while cross compiling is quite logical if you
know what the check AC_FUNC_MALLOC is made for. The behavior isn't
really wrong (for me) because autoconf can't really test the created
code due it's for another platform. Some nice explanation about the
whole problem can be found here:
http://wiki.buici.com/xwiki/bin/view/Programing+C+and+C%2B%2B/Autoconf+and+RPL_MALLOC

As long as libcoap uses malloc from the libc we need a check for
the proper implementation for this cases. So removing AC_FUNC_MALLOC
isn't solving the problem. We need at least a check for a malloc
functionality which can be done by AC_CHECK_FUNC(S).

(cherry picked from commit 04d806fe4d)

configure.ac

@@ -284,9 +284,8 @@ AC_TYPE_SIZE_T
 AC_TYPE_SSIZE_T
 
 # Checks for library functions.
-AC_FUNC_MALLOC
 AC_CHECK_FUNCS([memset select socket strcasecmp strrchr getaddrinfo \
-                strnlen])
+                strnlen malloc])
 
 # Check if -lsocket -lnsl is required (specifically Solaris)
 AC_SEARCH_LIBS([socket], [socket])