#16 Remove AC_FUNC_MALLOC check

[James]

Hi there,
When cross-compiling sshpass I came across this error on various musl/ARM platforms:

cc  -g -O2   -o sshpass main.o                                                                                                                                                                                                                 
Undefined symbols for architecture x86_64:                                                                                                                                                                                                     
  "_rpl_malloc", referenced from:                                                                                                                                                                                                              
      _runprogram in main.o                                                                                                                                                                                                                    
ld: symbol(s) not found for architecture x86_64                                                                                                                                                                                                
clang: error: linker command failed with exit code 1 (use -v to see invocation)                                                                                                                                                                
make[1]: *** [Makefile:417: sshpass] Error 1                                                                                                                                                                                                   
make[1]: Leaving directory '/workspace/srcdir/sshpass-1.10'                                                                                                                                                                                    
make: *** [Makefile:320: all] Error 2 

From googling it looks like it's because of the overzealous AC_FUNC_MALLOC check: https://github.com/maxmind/libmaxminddb/pull/152

Removing that line from configure.ac fixed the compilation issue for me, so I thought I'd submit a patch.