dev-libs/libsecp256k1: fix cross-compilation

See: https://github.com/bitcoin-core/secp256k1/pull/1159
Signed-off-by: Matt Whitlock <gentoo@mattwhitlock.name>
Closes: https://github.com/gentoo/gentoo/pull/28990
Signed-off-by: Sam James <sam@gentoo.org>

2 files changed, 90 insertions, 0 deletions

diff --git a/dev-libs/libsecp256k1/files/0.2.0-fix-cross-compile.patch b/dev-libs/libsecp256k1/files/0.2.0-fix-cross-compile.patch
new file mode 100644
index 000000000000..585e9b491a81
--- /dev/null
+++ b/dev-libs/libsecp256k1/files/0.2.0-fix-cross-compile.patch
@@ -0,0 +1,85 @@
+From 772e747bd9104d80fe531bed61f23f75342d7d63 Mon Sep 17 00:00:00 2001
+From: Matt Whitlock <bitcoin@mattwhitlock.name>
+Date: Sun, 20 Nov 2022 01:46:07 -0500
+Subject: [PATCH] Makefile: build precomp generators using build-system
+ toolchain
+
+When cross-compiling libsecp256k1, if the `precomputed_ecmult*.c` source
+files need to be regenerated, then the generators need to be built for
+the *build* system, not for the *host* system. Autoconf supports this
+fairly cleanly via the `AX_PROG_CC_FOR_BUILD` macro (from Autoconf
+Archive), but Automake requires some hackery. When building the
+generators, we override the `CC`, `CFLAGS`, `CPPFLAGS`, and `LDFLAGS`
+variables to their build-system counterparts, whose names are suffixed
+with `_FOR_BUILD` and whose values are populated by the aforementioned
+Autoconf macro and may be overridden on the `make` command line. Since
+Automake lacks support for overriding `EXEEXT` on a per-program basis,
+we define a recipe that builds the generator binaries with names
+suffixed with `$(EXEEXT)` and then renames them suffixed with
+`$(BUILD_EXEEXT)`.
+---
+ Makefile.am  | 30 ++++++++++++++++++++++++------
+ configure.ac |  1 +
+ 2 files changed, 25 insertions(+), 6 deletions(-)
+
+diff --git a/Makefile.am b/Makefile.am
+index 30b6a794d0..e929300298 100644
+--- a/Makefile.am
++++ b/Makefile.am
+@@ -180,8 +180,26 @@ endif
+ endif
+ 
+ ### Precomputed tables
+-EXTRA_PROGRAMS = precompute_ecmult precompute_ecmult_gen
+-CLEANFILES = $(EXTRA_PROGRAMS)
++PROGRAMS_FOR_BUILD = precompute_ecmult precompute_ecmult_gen
++$(addsuffix $(BUILD_EXEEXT),$(PROGRAMS_FOR_BUILD)) : override CC = $(CC_FOR_BUILD)
++$(addsuffix $(BUILD_EXEEXT),$(PROGRAMS_FOR_BUILD)) : override CFLAGS = $(CFLAGS_FOR_BUILD)
++$(addsuffix $(BUILD_EXEEXT),$(PROGRAMS_FOR_BUILD)) : override CPPFLAGS = $(CPPFLAGS_FOR_BUILD)
++$(addsuffix $(BUILD_EXEEXT),$(PROGRAMS_FOR_BUILD)) : override LDFLAGS = $(LDFLAGS_FOR_BUILD)
++# Automake has no support for PROGRAMS suffixed with BUILD_EXEEXT
++# instead of EXEEXT, so if those extensions differ, then we define a
++# recipe that builds the latter and renames it to the former. Since
++# Cygwin toolchains always append '.exe' to the output file name given
++# by '-o', we ignore rename failures since the toolchain will have
++# already created the right output file. (Note: The leading spaces
++# before ifneq and endif here are a hack so Automake won't try to
++# interpret them as an Automake conditional.)
++ ifneq ($(BUILD_EXEEXT),$(EXEEXT))
++%$(BUILD_EXEEXT) : %$(EXEEXT)
++	mv -- '$<' '$@' || :
++ endif
++
++EXTRA_PROGRAMS = $(PROGRAMS_FOR_BUILD)
++CLEANFILES = $(addsuffix $(BUILD_EXEEXT),$(PROGRAMS_FOR_BUILD))
+ 
+ precompute_ecmult_SOURCES = src/precompute_ecmult.c
+ precompute_ecmult_CPPFLAGS = $(SECP_INCLUDES)
+@@ -198,11 +216,11 @@ precompute_ecmult_gen_LDADD = $(SECP_LIBS) $(COMMON_LIB)
+ # This means that rebuilds of the prebuilt files always need to be
+ # forced by deleting them, e.g., by invoking `make clean-precomp`.
+ src/precomputed_ecmult.c:
+-	$(MAKE) $(AM_MAKEFLAGS) precompute_ecmult$(EXEEXT)
+-	./precompute_ecmult$(EXEEXT)
++	$(MAKE) $(AM_MAKEFLAGS) precompute_ecmult$(BUILD_EXEEXT)
++	./precompute_ecmult$(BUILD_EXEEXT)
+ src/precomputed_ecmult_gen.c:
+-	$(MAKE) $(AM_MAKEFLAGS) precompute_ecmult_gen$(EXEEXT)
+-	./precompute_ecmult_gen$(EXEEXT)
++	$(MAKE) $(AM_MAKEFLAGS) precompute_ecmult_gen$(BUILD_EXEEXT)
++	./precompute_ecmult_gen$(BUILD_EXEEXT)
+ 
+ PRECOMP = src/precomputed_ecmult_gen.c src/precomputed_ecmult.c
+ precomp: $(PRECOMP)
+diff --git a/configure.ac b/configure.ac
+index a2a15d2b82..013964f5ff 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -35,6 +35,7 @@ m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
+ AC_PROG_CC
+ AM_PROG_AS
+ AM_PROG_AR
++AX_PROG_CC_FOR_BUILD
+ 
+ # Clear some cache variables as a workaround for a bug that appears due to a bad
+ # interaction between AM_PROG_AR and LT_INIT when combining MSVC's archiver lib.exe.
diff --git a/dev-libs/libsecp256k1/libsecp256k1-0.2.0-r2.ebuild b/dev-libs/libsecp256k1/libsecp256k1-0.2.0-r2.ebuild
index 9391a6c99236..588001fd95a4 100644
--- a/dev-libs/libsecp256k1/libsecp256k1-0.2.0-r2.ebuild
+++ b/dev-libs/libsecp256k1/libsecp256k1-0.2.0-r2.ebuild
@@ -21,10 +21,15 @@ REQUIRED_USE="
 	schnorr? ( extrakeys )
 "
 BDEPEND="
+	sys-devel/autoconf-archive
 	virtual/pkgconfig
 	valgrind? ( dev-util/valgrind )
 "
 
+PATCHES=(
+	"${FILESDIR}/0.2.0-fix-cross-compile.patch"
+)
+
 S="${WORKDIR}/${MyPN}-${PV}"
 
 src_prepare() {