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/field.h

@@ -18,10 +18,6 @@
  *    imply normality.
  */
 
-#if defined HAVE_CONFIG_H
-#include "libsecp256k1-config.h"
-#endif
-
 #include "util.h"
 
 #if defined(SECP256K1_WIDEMUL_INT128)