Merge bitcoin-core/secp256k1#936: Fix gen_context/ASM build on ARM

c848352 Makefile.am: Don't pass a variable twice (Tim Ruffing)
2161f31 Makefile.am: Honor config when building gen_context (Tim Ruffing)
99f47c2 gen_context: Don't use external ASM because it complicates the build (Tim Ruffing)

Pull request description:

  Obsoletes bitcoin#935.

ACKs for top commit:
  gmaxwell:
    ACK c848352   looks good and works here. Undefign is kinda yuck, but it is already doing it and it's cleaner than the obvious alternatives.
  sipa:
    utACK c848352. I verified that building still works on ARM64, but without asm of course.

Tree-SHA512: fc5500688b2aecc4238e21c32f65559bcbfd1e83d1ae4d2c8e15573e94613667731064d8b5f2b9e4209016d88118263802ff4b9a73c1f37c224ccf2a4a1d6536

Makefile.am

@@ -101,7 +101,7 @@ if VALGRIND_ENABLED
 tests_CPPFLAGS += -DVALGRIND
 noinst_PROGRAMS += valgrind_ctime_test
 valgrind_ctime_test_SOURCES = src/valgrind_ctime_test.c
-valgrind_ctime_test_LDADD = libsecp256k1.la $(SECP_LIBS) $(SECP_LIBS) $(COMMON_LIB)
+valgrind_ctime_test_LDADD = libsecp256k1.la $(SECP_LIBS) $(COMMON_LIB)
 endif
 if !ENABLE_COVERAGE
 tests_CPPFLAGS += -DVERIFY
@@ -129,7 +129,7 @@ CPPFLAGS_FOR_BUILD +=-I$(top_srcdir) -I$(builddir)/src
 gen_context_OBJECTS = gen_context.o
 gen_context_BIN = gen_context$(BUILD_EXEEXT)
 gen_%.o: src/gen_%.c src/libsecp256k1-config.h
-	$(CC_FOR_BUILD) $(CPPFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
+	$(CC_FOR_BUILD) $(DEFS) $(CPPFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
 
 $(gen_context_BIN): $(gen_context_OBJECTS)
 	$(CC_FOR_BUILD) $(CFLAGS_FOR_BUILD) $(LDFLAGS_FOR_BUILD) $^ -o $@

src/gen_context.c

@@ -13,6 +13,11 @@
 /* We can't require the precomputed tables when creating them. */
 #undef USE_ECMULT_STATIC_PRECOMPUTATION
 
+/* In principle we could use external ASM, but this yields only a minor speedup in
+   build time and it's very complicated. In particular when cross-compiling, we'd
+   need to build the external ASM for the build and the host machine. */
+#undef USE_EXTERNAL_ASM
+
 #include "include/secp256k1.h"
 #include "assumptions.h"
 #include "util.h"