wally: Specify that we use python3 so we don't depend on python2

We specify `PYTHON_VERSION=3` to prevent libwally's ./configure from searchin
for python2, which some distros have started removing, and we were requiring
it only for the configuration step anyway.

Changelog-Changed: dependencies: We no longer depend on python2 which has reached end-of-life

external/Makefile

@@ -63,10 +63,15 @@ $(LIBWALLY_HEADERS) $(LIBSECP_HEADERS): submodcheck
 $(TARGET_DIR)/libsecp256k1.% $(TARGET_DIR)/libwallycore.%: $(TARGET_DIR)/libwally-core-build/src/secp256k1/libsecp256k1.la $(TARGET_DIR)/libwally-core-build/src/libwallycore.la
 	$(MAKE) -C $(TARGET_DIR)/libwally-core-build DESTDIR=$$(pwd)/$(TARGET_DIR) install-exec
 
+# Build libwally-core.
+#
+# PYTHON_VERSION is required to prevent configure from complaining about
+# missing python2 binaries. These have been removed in newer distros and we
+# shouldn't require them to be present either.
 $(TARGET_DIR)/libwally-core-build/src/libwallycore.% $(TARGET_DIR)/libwally-core-build/src/secp256k1/libsecp256k1.%: $(LIBWALLY_HEADERS) $(LIBSECP_HEADERS)
 	cd external/libwally-core && ./tools/autogen.sh
 	mkdir -p ${TARGET_DIR}/libwally-core-build
-	cd ${TARGET_DIR}/libwally-core-build && ${TOP}/libwally-core/configure CC="$(CC)" --enable-static=yes $(CROSSCOMPILE_OPTS) --enable-module-recovery --enable-elements --enable-shared=no --prefix=/ --libdir=/ --enable-debug && $(MAKE)
+	cd ${TARGET_DIR}/libwally-core-build && PYTHON_VERSION=3 ${TOP}/libwally-core/configure CC="$(CC)" --enable-static=yes $(CROSSCOMPILE_OPTS) --enable-module-recovery --enable-elements --enable-shared=no --prefix=/ --libdir=/ --enable-debug && $(MAKE)
 
 external/jsmn/jsmn.h: submodcheck