configure.in: Remove AC_FUNC_MALLOC

The check for malloc usually is not required but can fail when
cross-compiling. In that case, autoconf looks for rpl_malloc()
which would need to be implemented by the application. A common
way of dealing with this is to remove AC_FUNC_MALLOC from
configure and have the user provide a malloc() wrapper if required.

Change-Id: I24ab688d0ffdcf7233db9f250a5d8255d72a9ad4

configure.ac

diff --git a/configure.ac b/configure.ac
index 5a76a1e..ae6a3f9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -76,7 +76,6 @@
 		[#include <netinet/in.h>])
 
 # Checks for library functions.
-AC_FUNC_MALLOC
 AC_CHECK_FUNCS([memset select socket strdup strerror strnlen fls vprintf])
 
 AC_CONFIG_HEADERS([dtls_config.h])