Mocking library functions in C

I want to mock functions of the c lib such as malloc without altering too much the original source file (the one that uses malloc).

I tried including a header file "mock.h" like

#ifndef MOCK_H_
# define MOCK_H_
# ifdef MOCK_MODE

#  include <sys/types.h>

extern void *my_mock_malloc(size_t n);
void    *malloc(size_t n) __attribute__((weak, alias ("my_mock_malloc")));

# endif /* MOCK_MODE */
#endif /* !MOCK_H_ */

but it gives me an error

in file included from ...:
/usr/include/stdlib.h:466:14: error: ‘malloc’ aliased to undefined symbol ‘my_mock_malloc’
 extern void *malloc (size_t __size) __THROW __attribute_malloc__ __wur;

GCC alias to function outside of translation unit -AKA- is this even the right tool for the job? gives a partial solution: using the linker I create an alias on a symbol. I can now compile with -Xlinker --defsym "malloc"="my_mock_malloc". The problem is that all my .o files are linked using this option and thus the unit testing framework I use (check) is affected by the mocking (and thus it receives is SIGSEGV when I make my mock function return NULL).

Is there a way to perform such symbol aliasing locally, so I can make my tesing framework use the real malloc? Os is there a better solution than this one?