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/precomputed_ecmult_gen.c

@@ -1,8 +1,5 @@
 /* This file was automatically generated by precompute_ecmult_gen. */
 /* See ecmult_gen_impl.h for details about the contents of this file. */
-#if defined HAVE_CONFIG_H
-#    include "libsecp256k1-config.h"
-#endif
 #include "../include/secp256k1.h"
 #include "group.h"
 #include "ecmult_gen.h"