Remove dependency on src/libsecp256k1-config.h

This change eases the use of alternate build systems by moving
the variables in `src/libsecp256k1-config.h` to compiler macros
for each invocation, preventing duplication of these variables
for each build system.

Co-authored-by: Ali Sherief <ali@notatether.com>

src/tests.c

@@ -4,10 +4,6 @@
  * file COPYING or https://www.opensource.org/licenses/mit-license.php.*
  ***********************************************************************/
 
-#if defined HAVE_CONFIG_H
-#include "libsecp256k1-config.h"
-#endif
-
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>