make: Explicitly set PYTHON_VERSION=3 for libwally

Otherwise it can end up complaining that it can't find python2, which
we don't have listed as a dependency and is deprecated anyway.

Also reformatted a bit to make the options more readable and minimize
future conflicts.

external/Makefile

@@ -71,7 +71,19 @@ $(TARGET_DIR)/libsecp256k1.% $(TARGET_DIR)/libwallycore.%: $(TARGET_DIR)/libwall
 $(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 && CFLAGS=-std=c99 ${TOP}/libwally-core/configure CC="$(CC)" --enable-static=yes $(CROSSCOMPILE_OPTS) --enable-module-recovery --enable-module-extrakeys --enable-module-schnorrsig --enable-elements --enable-shared=no --prefix=/ --libdir=/ --enable-debug && $(MAKE)
+	cd ${TARGET_DIR}/libwally-core-build \
+	&& PYTHON_VERSION=3 CFLAGS=-std=c99 ${TOP}/libwally-core/configure CC="$(CC)" \
+		--enable-static=yes \
+		$(CROSSCOMPILE_OPTS) \
+		--enable-module-recovery \
+		--enable-module-extrakeys \
+		--enable-module-schnorrsig \
+		--enable-elements \
+		--enable-shared=no \
+		--prefix=/ \
+		--libdir=/ \
+		--enable-debug \
+	&& $(MAKE)
 
 # If we tell Make that the above builds both, it runs it twice in
 # parallel.  So we lie :(