configure.ac: Don't check for malloc, realloc

When cross-compiling for Android, configure (erroneously?) decides we don't
have a GNU-compatible malloc, and tries to

    #define malloc rpl_malloc

We don't define rpl_malloc, and it's not clear it would work with the <cstdlib>
C++ header, anyway.  See

    http://nerdland.net/unstumping-the-internet/malloc-has-not-been-declared/

If we aren't using malloc(0), it should be safe to use malloc without the check.

configure.ac

@@ -110,9 +110,7 @@ AC_TYPE_UINT8_T
 
 # Checks for library functions.
 AC_FUNC_FORK
-AC_FUNC_MALLOC
 AC_FUNC_MBRTOWC
-AC_FUNC_REALLOC
 AC_CHECK_FUNCS([gettimeofday setrlimit inet_ntoa iswprint memchr memset nl_langinfo posix_memalign setenv setlocale sigaction socket strchr strdup strerror strtol wcwidth])
 
 AC_SEARCH_LIBS([clock_gettime], [rt], [AC_DEFINE([HAVE_CLOCK_GETTIME], [1], [Define if clock_gettime is available.])])