Another Darwin unbreak

[Jason Bacon <outpaddling%yahoo.com@localhost>]

Another major update was just released, bumping the OS version to 11.5, 
but still called "Big Sur".

Any objections to the following quick fix?

I wiped and reinstalled the command line tools to verify that there is 
nothing newer than 11.3.

--- platform/Darwin.mk	27 May 2021 18:16:26 -0000	1.103
+++ platform/Darwin.mk	25 Jul 2021 00:46:45 -0000
@@ -25,8 +25,8 @@
 # High Sierra	10.13.x	17.x.y	9.3  (llvm clang 9.0)
 # Mojave	10.14.x	18.x.y	10.2 (llvm clang 10.0)
 # Catalina	10.15.x 19.x.y
-# Big Sur	11.0*   20.0    12.0 (llvm clang 12.0)
-#  *) 11.0 on ARM, 10.16 on Intel Macs, for now
+# Big Sur	11.x*   20.x.y  12.0 (llvm clang 12.0)
+#  *) 11.0 on ARM, 10.16 on Intel Macs initially

 # Tiger (and earlier) use Xfree 4.4.0 (and earlier)
 .if empty(MACHINE_PLATFORM:MDarwin-[0-8].*-*)
@@ -111,6 +111,7 @@
 #
 OSX_SDK_MAP.11.2=	11.1
 OSX_SDK_MAP.11.4=	11.3
+OSX_SDK_MAP.11.5=	11.3
 #
 OSX_SDK_PATH!=	/usr/bin/xcrun \
 		    --sdk macosx${OSX_SDK_MAP.${OSX_VERSION}:U${OSX_VERSION}} \