Windows build improvements (#46793)

* Include unistd.h to silence compilation warning.

* Error when running in a MSYS shell.

Make.inc

@@ -772,6 +772,8 @@ else ifeq (cygwin, $(shell $(CC) -dumpmachine | cut -d\- -f3))
 $(error "cannot build julia with cygwin-target compilers. set XC_HOST to i686-w64-mingw32 or x86_64-w64-mingw32 for mingw cross-compile")
 else ifeq (msys, $(shell $(CC) -dumpmachine | cut -d\- -f3))
 $(error "cannot build julia with msys-target compilers. please see the README.windows document for instructions on setting up mingw-w64 compilers")
+else ifneq (,$(findstring MSYS,$(shell uname)))
+$(error "cannot build julia from a msys shell. please launch a mingw shell instead by setting MSYSTEM=MINGW64")
 endif
 
 ifeq ($(BUILD_OS),Darwin)

src/signal-handling.c

@@ -6,8 +6,8 @@
 #include <inttypes.h>
 #include "julia.h"
 #include "julia_internal.h"
-#ifndef _OS_WINDOWS_
 #include <unistd.h>
+#ifndef _OS_WINDOWS_
 #include <sys/mman.h>
 #endif