nix: fix md5sum on MacOS

The default on MacOS is `md5`, while Nix only has `md5sum` available.
Therefore, make it possible to override the variable via the environment
so that flake.nix can set the correct binary name.

GNUmakefile

@@ -32,9 +32,6 @@ export GOROOT = $(shell $(GO) env GOROOT)
 # Flags to pass to go test.
 GOTESTFLAGS ?=
 
-# md5sum binary
-MD5SUM = md5sum
-
 # tinygo binary for tests
 TINYGO ?= $(call detect,tinygo,tinygo $(CURDIR)/build/tinygo)
 
@@ -130,14 +127,14 @@ ifeq ($(OS),Windows_NT)
     USE_SYSTEM_BINARYEN ?= 1
 
 else ifeq ($(shell uname -s),Darwin)
-    MD5SUM = md5
+    MD5SUM ?= md5
 
     CGO_LDFLAGS += -lxar
 
     USE_SYSTEM_BINARYEN ?= 1
 
 else ifeq ($(shell uname -s),FreeBSD)
-    MD5SUM = md5
+    MD5SUM ?= md5
     START_GROUP = -Wl,--start-group
     END_GROUP = -Wl,--end-group
 else
@@ -145,6 +142,9 @@ else
     END_GROUP = -Wl,--end-group
 endif
 
+# md5sum binary default, can be overridden by an environment variable
+MD5SUM ?= md5sum
+
 # Libraries that should be linked in for the statically linked Clang.
 CLANG_LIB_NAMES = clangAnalysis clangAST clangASTMatchers clangBasic clangCodeGen clangCrossTU clangDriver clangDynamicASTMatchers clangEdit clangExtractAPI clangFormat clangFrontend clangFrontendTool clangHandleCXX clangHandleLLVM clangIndex clangLex clangParse clangRewrite clangRewriteFrontend clangSema clangSerialization clangSupport clangTooling clangToolingASTDiff clangToolingCore clangToolingInclusions
 CLANG_LIBS = $(START_GROUP) $(addprefix -l,$(CLANG_LIB_NAMES)) $(END_GROUP) -lstdc++

flake.nix

@@ -72,6 +72,10 @@
             export LLVM_AR=llvm-ar
             export LLVM_NM=llvm-nm
 
+            # Make `make smoketest` work (the default is `md5`, while Nix only
+            # has `md5sum`).
+            export MD5SUM=md5sum
+
             # Ugly hack to make the Clang resources directory available.
             export GOFLAGS="\"-ldflags=-X github.com/tinygo-org/tinygo/goenv.clangResourceDir=${llvmPackages_16.clang.cc.lib}/lib/clang/16"\"
           '';