gen_context: Don't include basic-config.h

Before this commit, gen_context.c both included libsecp256k1-config.h
and basic-config.h: The former only to obtain ECMULT_GEN_PREC_BITS
and the latter to obtain a basic working configuration to be able to
use the library.

This was inelegant and confusing: It meant that basic-config.h needs
to #undef all the macros defined in libsecp256k1-config.h. Moreover,
it meant that basic-config.h cannot define ECMULT_GEN_PREC_BITS,
essentially making this file specific for use in gen_context.c.

After this commit, gen_context.c include only libsecp256k1-config.h.
basic-config.h is not necessary anymore for the modules used in
gen_context.c because 79f1f7a made the preprocessor detect all the
relevant config options.

On the way, we remove an unused #define in basic-config.h.

src/basic-config.h

@@ -9,15 +9,6 @@
 
 #ifdef USE_BASIC_CONFIG
 
-#undef USE_ASM_X86_64
-#undef USE_ECMULT_STATIC_PRECOMPUTATION
-#undef USE_EXTERNAL_ASM
-#undef USE_EXTERNAL_DEFAULT_CALLBACKS
-#undef USE_FORCE_WIDEMUL_INT64
-#undef USE_FORCE_WIDEMUL_INT128
-#undef ECMULT_WINDOW_SIZE
-
-#define USE_WIDEMUL_64 1
 #define ECMULT_WINDOW_SIZE 15
 
 #endif /* USE_BASIC_CONFIG */

src/gen_context.c

@@ -9,8 +9,9 @@
 #if !defined(ECMULT_GEN_PREC_BITS)
 #include "libsecp256k1-config.h"
 #endif
-#define USE_BASIC_CONFIG 1
-#include "basic-config.h"
+
+/* We can't require the precomputed tables when creating them. */
+#undef USE_ECMULT_STATIC_PRECOMPUTATION
 
 #include "include/secp256k1.h"
 #include "assumptions.h"