Set the host and target environment types at configuration time.

Branches: main, 11.07

Set the host and target environment types at configuration time.  By default,
they were both set to "posix" but that is no longer compatible with building
the csharp grade using the Microsoft compiler.

configure.in:
	Set the environment type to "cygwin" on Cygwin, to "msys" on MinGW
	and to "posix" otherwise.  (For Mercury installations that work with
	the Windows command interpreter, it will need to be manually set to
	"windows".)

scripts/Mercury.config.in:
	Set the environment type.  (We set the target and host separately
	so that is easier for installers to automatically modify this file.)

README.MS-VisualC:
	Conform to the above change.

README.MS-VisualC

@@ -157,9 +157,12 @@ directory
    
   + Replace any Unix-style paths with their Windows-style equivalent. 
 
-  + Add the following option in the value of the variable DEFAULT_MCFLAGS
- 
-    --env-type windows
+  + Modify the values of the options --host-env-type and --target-env-type
+    in the value of the variable DEFAULT_MCFLAGS so that their values are as
+    follows:
+
+    --host-env-type "windows"
+    --target-env-type "windows"
 
   + To specify the commands used to install files and directories include
     the following options in the value of the variable DEFAULT_MCFLAGS

configure.in

@@ -4639,12 +4639,18 @@ esac
 case "$host" in
     *-cygwin*)
         EXT_FOR_EXE=".exe"
+        HOST_ENV_TYPE=cygwin
+        TARGET_ENV_TYPE=cygwin
         ;;
     *mingw*)
         EXT_FOR_EXE=".exe"
+        HOST_ENV_TYPE=msys
+        TARGET_ENV_TYPE=msys
         ;;
     *)
         EXT_FOR_EXE=
+        HOST_ENV_TYPE=posix
+        TARGET_ENV_TYPE=posix
         ;;
 esac
 
@@ -4668,6 +4674,8 @@ AC_SUBST(EXT_FOR_PIC_OBJECTS)
 AC_SUBST(EXT_FOR_LINK_WITH_PIC_OBJECTS)
 AC_SUBST(EXT_FOR_SHARED_LIB)
 AC_SUBST(EXT_FOR_EXE)
+AC_SUBST(HOST_ENV_TYPE)
+AC_SUBST(TARGET_ENV_TYPE)
 AC_SUBST(USE_DLLS)
 AC_SUBST(SHARED_LIBS)
 AC_SUBST(SHARED_LIBS_SH)

scripts/Mercury.config.in

@@ -123,6 +123,8 @@ DEFAULT_MCFLAGS=\
 		--bytes-per-word "@BYTES_PER_WORD@" \
 		--sync-term-size "@SYNC_TERM_SIZE@" \
 		--dotnet-library-version "@MS_DOTNET_LIBRARY_VERSION@" \
+		--host-env-type "@HOST_ENV_TYPE@" \
+		--target-env-type "@TARGET_ENV_TYPE@" \
 		@RESTRICTED_COMMAND_LINE_OPT@ \
 		@HAVE_DELAY_SLOT@ \
 		@HAVE_BOXED_FLOATS@ \