added extra install instructions

README.md

@@ -33,6 +33,23 @@ to find it:
 
  env PG_CONFIG=/path/to/pg_config make && make install
 
+Some prepackaged Mac installs of postgres might need a little coaxing with
+modern XCodes. If you encounter an error such as:
+
+ make: /Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.8.xctoolchain/usr/bin/cc: No such file or directory
+
+Then you'll need to link the toolchain
+
+ sudo ln -s /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain /Applications/Xcode.app/Contents/Developer/Toolchains/OSX10.8.xctoolchain
+
+And if you encounter an error about a missing `/usr/bin/postgres`:
+
+ ld: file not found: /usr/bin/postgres
+
+You might need to link in your real postgres:
+
+ sudo ln -s /usr/bin/postgres_real /usr/bin/postgres
+
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