Makefile: don't cross-compile executor if cross-compiler is missing

We started building netbsd executor on linux.
This fails on CI since cross-compiler is missing.
But we don't want to exclude netbsd from presubmit entirely.
So check if cross-compiler is missing and just produce a warning.
This should unbreak CI and humans will see the warning.

Makefile

@@ -110,14 +110,20 @@ target:
 
 # executor uses stacks of limited size, so no jumbo frames.
 executor:
-ifeq ($(BUILDOS),$(NATIVEBUILDOS))
-	mkdir -p ./bin/$(TARGETOS)_$(TARGETARCH)
-	$(CC) -o ./bin/$(TARGETOS)_$(TARGETARCH)/syz-executor$(EXE) executor/executor.cc \
-		$(ADDCFLAGS) $(CFLAGS) -DGOOS_$(TARGETOS)=1 -DGOARCH_$(TARGETARCH)=1  -DGIT_REVISION=\"$(REV)\"
-else
+ifneq ("$(BUILDOS)", "$(NATIVEBUILDOS)")
 	$(info ************************************************************************************)
 	$(info Building executor for ${TARGETOS} is not supported on ${BUILDOS}. Executor will not be built.)
 	$(info ************************************************************************************)
+else
+ifneq ("$(NO_CROSS_COMPILER)", "")
+	$(info ************************************************************************************)
+	$(info Native cross-compiler $(CC) is missing. Executor will not be built.)
+	$(info ************************************************************************************)
+else
+	mkdir -p ./bin/$(TARGETOS)_$(TARGETARCH)
+	$(CC) -o ./bin/$(TARGETOS)_$(TARGETARCH)/syz-executor$(EXE) executor/executor.cc \
+		$(ADDCFLAGS) $(CFLAGS) -DGOOS_$(TARGETOS)=1 -DGOARCH_$(TARGETARCH)=1  -DGIT_REVISION=\"$(REV)\"
+endif
 endif
 
 manager:

tools/syz-env/env.go

@@ -6,6 +6,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"runtime"
 	"strconv"
 	"strings"
@@ -40,6 +41,12 @@ func main() {
 		{"ADDCFLAGS", strings.Join(target.CrossCFlags, " ")},
 		{"NCORES", strconv.Itoa(runtime.NumCPU())},
 		{"EXE", target.ExeExtension},
+		{"NATIVEBUILDOS", target.BuildOS},
+	}
+	if targetOS != runtime.GOOS {
+		if _, err := exec.LookPath(target.CCompiler); err != nil {
+			vars = append(vars, Var{"NO_CROSS_COMPILER", "yes"})
+		}
 	}
 	for _, v := range vars {
 		fmt.Printf("export %v=%v\\n", v.Name, v.Val)