give makefile a concept of cross compiling, use conditionals to determin what it is, and if it can

Makefile

@@ -7,8 +7,17 @@ DEFAULT_GOAL: linux
 
 all: linux android windows
 
+ARCH := $(shell uname -m)
+ARM_X_CC := aarch64-linux-gnu-gcc
+
+# determine if the x64 cross arm64 compiler is available so we can add that target
+LINUX_X_ARM=
+ifneq (,$(shell which $(ARM_X_CC)))
+	LINUX_X_ARM = libCwtch.x.so
+endif
+
 linux: EXPERIMENTS ?= serverExperiment
-linux: libCwtch.x64.so libCwtch.arm64.so
+linux: libCwtch.so $(LINUX_X_ARM)
 
 macos: EXPERIMENTS ?= serverExperiment
 macos: libCwtch.x64.dylib libCwtch.arm64.dylib
@@ -20,24 +29,29 @@ windows: EXPERIMENTS ?= serverExperiment
 windows: libCwtch.dll
 
 
-
 lib.go: generate/generate_bindings.go spec
 	go run generate/generate_bindings.go --experiments "$(EXPERIMENTS)"
 
 
-libCwtch.x64.so: lib.go
+libCwtch.so: lib.go
 	./switch-ffi.sh
-	go build -trimpath -ldflags "-buildid=autobindings-$(shell git describe --tags) -X main.buildVer=autobindings-$(shell git describe --tags) -X main.buildDate=$(shell git log -1 --format=%cd --date=format:%G-%m-%d-%H-%M)" -buildmode c-shared -o libCwtch.x64.so
+	go build -trimpath -ldflags "-buildid=autobindings-$(shell git describe --tags) -X main.buildVer=autobindings-$(shell git describe --tags) -X main.buildDate=$(shell git log -1 --format=%cd --date=format:%G-%m-%d-%H-%M)" -buildmode c-shared -o libCwtch.so
 	mkdir -p build/linux
-	mv libCwtch.x64.so build/linux/
-	mv libCwtch.x64.h build/linux/libCwtch.h
-
-libCwtch.arm64.so:
+ifeq ($(ARCH),x86_64)
+	mv libCwtch.so build/linux/libCwtch.x64.so
+else ifeq ($(ARCH),aarch64)
+	mv libCwtch.so build/linux/libCwtch.arm64.so
+endif
+	mv libCwtch.h build/linux/libCwtch.h
+
+libCwtch.x.so: lib.go
 	./switch-ffi.sh
-	env CGO_ENABLED=1 CC=aarch64-linux-gnu-gcc  GOARCH=arm64 go build -trimpath -ldflags "-buildid=autobindings-v0.0.3-22-g20065b2 -X main.buildVer=autobindings-v0.0.3-22-g20065b2 -X main.buildDate=2023-05-01-21-41" -buildmode c-shared -o libCwtch.arm64.so
 	mkdir -p build/linux
-	mv libCwtch.arm64.so build/linux/
-	mv libCwtch.arm64.h build/linux/libCwtch.h
+ifeq ($(ARCH),x86_64)
+	env CGO_ENABLED=1 CC=$(ARM_X_CC)  GOARCH=arm64 go build -trimpath -ldflags "-buildid=autobindings-v0.0.3-22-g20065b2 -X main.buildVer=autobindings-v0.0.3-22-g20065b2 -X main.buildDate=2023-05-01-21-41" -buildmode c-shared -o libCwtch.x.so
+	mv libCwtch.x.so build/linux/libCwtch.arm64.so
+	mv libCwtch.x.h build/linux/libCwtch.h
+endif
 
 libCwtch.x64.dylib: lib.go
 	./switch-ffi.sh

README.md

@@ -59,6 +59,10 @@ Other directives:
 - `LD_LIBRARY_PATH` set to point to `libCwtch.so`
 - or drop a symlink into `/usr/lib`
 
+### Cross compile arm 64
+
+Needs `sudo apt install gcc-aarch64-linux-gnu`
+
 ## Android
 
 - copy `cwtch.aar` into `flutter_app/android/cwtch`