nix: improve docs and add support for wasi-libc

I forgot a few things in the flake file, but now everything should be
included.

GNUmakefile

@@ -25,6 +25,11 @@ CLANG ?= $(call findLLVMTool,clang)
 LLVM_AR ?= $(call findLLVMTool,llvm-ar)
 LLVM_NM ?= $(call findLLVMTool,llvm-nm)
 
+# Patch up Clang for Nix.
+ifneq (, ${CLANG_RESOURCE_DIR})
+    CLANG := $(CLANG) -resource-dir ${CLANG_RESOURCE_DIR}
+endif
+
 # Go binary and GOROOT to select
 GO ?= go
 export GOROOT = $(shell $(GO) env GOROOT)
@@ -265,7 +270,7 @@ endif
 wasi-libc: lib/wasi-libc/sysroot/lib/wasm32-wasi/libc.a
 lib/wasi-libc/sysroot/lib/wasm32-wasi/libc.a:
 	@if [ ! -e lib/wasi-libc/Makefile ]; then echo "Submodules have not been downloaded. Please download them using:\n  git submodule update --init"; exit 1; fi
-	cd lib/wasi-libc && $(MAKE) -j4 EXTRA_CFLAGS="-O2 -g -DNDEBUG -mnontrapping-fptoint -msign-ext" MALLOC_IMPL=none CC=$(CLANG) AR=$(LLVM_AR) NM=$(LLVM_NM)
+	cd lib/wasi-libc && $(MAKE) -j4 EXTRA_CFLAGS="-O2 -g -DNDEBUG -mnontrapping-fptoint -msign-ext" MALLOC_IMPL=none CC="$(CLANG)" AR=$(LLVM_AR) NM=$(LLVM_NM)
 
 # Check for Node.js used during WASM tests.
 NODEJS_VERSION := $(word 1,$(subst ., ,$(shell node -v | cut -c 2-)))

flake.nix

@@ -16,6 +16,17 @@
 #
 #   tinygo version
 #
+# But you'll need a bit more to make TinyGo actually able to compile code:
+#
+#   make llvm-source              # fetch compiler-rt
+#   git submodule update --init   # fetch lots of other libraries and SVD files
+#   make gen-device -j4           # build src/device/*/*.go files
+#   make wasi-libc                # build support for wasi/wasm
+#
+# With this, you should have an environment that can compile anything - except
+# for the Xtensa architecture (ESP8266/ESP32) because support for that lives in
+# a separate LLVM fork.
+#
 # You can also do many other things from this environment. Building and flashing
 # should work as you're used to: it's not a VM or container so there are no
 # access restrictions and you're running in the same host environment - just
@@ -55,7 +66,8 @@
           shellHook= ''
             # Ugly hack to make the Clang resources directory available.
             # Perhaps there is a cleaner way to do it, but this works.
-            export GOFLAGS="\"-ldflags=-X github.com/tinygo-org/tinygo/goenv.clangResourceDir=${llvmPackages_16.clang.cc.lib}/lib/clang/16"\"
+            export CLANG_RESOURCE_DIR="${llvmPackages_16.clang.cc.lib}/lib/clang/16"
+            export GOFLAGS="\"-ldflags=-X github.com/tinygo-org/tinygo/goenv.clangResourceDir=$CLANG_RESOURCE_DIR"\"
           '';
         };
       }