build: Prebuild and distribute ecmult_gen table

 - Improve Makefile.am for both prebuilt tables files
 - On the way, tidy EXTRA_DIST: Move the header files to noinst_HEADERS,
   where they conceptually belong, and add missing SECURITY.md to EXTRA_DIST

Author: real-or-random

.gitattributes

@@ -1 +1,2 @@
 src/ecmult_static_pre_g.h linguist-generated
+src/ecmult_gen_static_prec_table.h linguist-generated

.gitignore

@@ -41,7 +41,6 @@ coverage.*.html
 
 src/libsecp256k1-config.h
 src/libsecp256k1-config.h.in
-src/ecmult_static_context.h
 build-aux/config.guess
 build-aux/config.sub
 build-aux/depcomp

Makefile.am

@@ -52,6 +52,7 @@ noinst_HEADERS += src/hash_impl.h
 noinst_HEADERS += src/field.h
 noinst_HEADERS += src/field_impl.h
 noinst_HEADERS += src/bench.h
+noinst_HEADERS += src/basic-config.h
 noinst_HEADERS += contrib/lax_der_parsing.h
 noinst_HEADERS += contrib/lax_der_parsing.c
 noinst_HEADERS += contrib/lax_der_privatekey_parsing.h
@@ -125,37 +126,37 @@ exhaustive_tests_LDFLAGS = -static
 TESTS += exhaustive_tests
 endif
 
-EXTRA_PROGRAMS = gen_ecmult_static_pre_g
+### Precomputed tables
+EXTRA_PROGRAMS = gen_ecmult_static_pre_g gen_ecmult_gen_static_prec_table
+CLEANFILES = $(EXTRA_PROGRAMS)
 gen_ecmult_static_pre_g_SOURCES = src/gen_ecmult_static_pre_g.c
-# See Automake manual, Section "Errors with distclean"
+gen_ecmult_gen_static_prec_table_SOURCES = src/gen_ecmult_gen_static_prec_table.c
+
+# See Automake manual, Section "Errors with distclean".
+# We don't list any dependencies for the prebuilt files here because
+# otherwise make's decision whether to rebuild them (even in the first
+# build by a normal user) depends on mtimes, and thus is very fragile.
+# This means that rebuilds of the prebuilt files always need to be
+# forced by deleting them, e.g., by invoking `make clean-precomp`.
 src/ecmult_static_pre_g.h:
 	$(MAKE) $(AM_MAKEFLAGS) gen_ecmult_static_pre_g$(EXEEXT)
 	./gen_ecmult_static_pre_g$(EXEEXT)
+src/ecmult_gen_static_prec_table.h:
+	$(MAKE) $(AM_MAKEFLAGS) gen_ecmult_gen_static_prec_table$(EXEEXT)
+	./gen_ecmult_gen_static_prec_table$(EXEEXT)
 
-if USE_ECMULT_STATIC_PRECOMPUTATION
-CPPFLAGS_FOR_BUILD +=-I$(top_srcdir) -I$(builddir)/src
+PRECOMP = src/ecmult_gen_static_prec_table.h src/ecmult_static_pre_g.h
+noinst_HEADERS += $(PRECOMP)
+precomp: $(PRECOMP)
 
-gen_ecmult_gen_static_prec_table_OBJECTS = gen_ecmult_gen_static_prec_table.o
-gen_ecmult_gen_static_prec_table_BIN = gen_ecmult_gen_static_prec_table$(BUILD_EXEEXT)
-$(gen_ecmult_gen_static_prec_table_OBJECTS): src/gen_ecmult_gen_static_prec_table.c src/libsecp256k1-config.h
-	$(CC_FOR_BUILD) $(DEFS) $(CPPFLAGS_FOR_BUILD) $(SECP_CFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) -c $< -o $@
+# One can use `make maintainerclean` instead of `make clean-precomp && make clean`.
+maintainerclean-local: clean-precomp
 
-$(gen_ecmult_gen_static_prec_table_BIN): $(gen_ecmult_gen_static_prec_table_OBJECTS)
-	$(CC_FOR_BUILD) $(SECP_CFLAGS_FOR_BUILD) $(CFLAGS_FOR_BUILD) $(LDFLAGS_FOR_BUILD) $^ -o $@
+.PHONY: clean-precomp
+clean-precomp:
+	rm -f $(PRECOMP)
 
-$(libsecp256k1_la_OBJECTS): src/ecmult_gen_static_prec_table.h
-$(tests_OBJECTS): src/ecmult_gen_static_prec_table.h
-$(exhaustive_tests_OBJECTS): src/ecmult_gen_static_prec_table.h
-$(bench_internal_OBJECTS): src/ecmult_gen_static_prec_table.h
-$(bench_ecmult_OBJECTS): src/ecmult_gen_static_prec_table.h
-
-src/ecmult_gen_static_prec_table.h: $(gen_ecmult_gen_static_prec_table_BIN)
-	./$(gen_ecmult_gen_static_prec_table_BIN)
-
-CLEANFILES = $(gen_ecmult_gen_static_prec_table_BIN) src/ecmult_gen_static_prec_table.h
-endif
-
-EXTRA_DIST = autogen.sh src/gen_ecmult_gen_static_prec_table.c src/ecmult_static_pre_g.h src/basic-config.h
+EXTRA_DIST = autogen.sh SECURITY.md
 
 if ENABLE_MODULE_ECDH
 include src/modules/ecdh/Makefile.am.include

configure.ac

@@ -125,9 +125,6 @@ AC_ARG_ENABLE(exhaustive_tests,
     [use_exhaustive_tests=$enableval],
     [use_exhaustive_tests=yes])
 
-# Force static precomputation. The code handling this will be removed in a later commit.
-use_ecmult_static_precomputation=yes
-
 AC_ARG_ENABLE(module_ecdh,
     AS_HELP_STRING([--enable-module-ecdh],[enable ECDH shared secret computation]),
     [enable_module_ecdh=$enableval],