add MacOS note

(cherry picked from commit 91e4700bccc1003b8f68cc52b926e8bb3e5c0869)

diff --git a/README.md b/README.md
index 302ce05920461a7a7a63c1aea90eeca36073130d..378233b7b189cfc11bf398629f6d4cdab9682ca6 100644 (file)
--- a/README.md
+++ b/README.md
@@ -278,6 +278,15 @@ affect compilation:
 |`BUILD_LDFLAGS`||
 |`BUILD_LIBS`||
 
+#### <a name="macos"> MacOS
+
+Building on MacOS assumes that the "Command Tools for Xcode" is installed.
+This can be downloaded from https://developer.apple.com/download/more/
+or if you have Xcode already installed you can run "Xcode-select --install".
+This will add /usr/include to the system and install the compiler and other
+tools so that they can be easily found.
+
+
 #### <a name="opts"/> Compile-time options
 
 To see a full list of configuration options, run `configure --help`.