Prevent clang from including /usr/local/include

Homebrew installs its headers and libraries in /usr/local. This means
that e.g. OpenSSL headers will be included from Homebrew instead of
those built by TeamTalk.

Output from clang -v:

_include "..." search starts here:
_include <...> search starts here:
 /Volumes/BEAR-1/MacOS/TeamTalk5/Library/TeamTalkLib
 /Volumes/BEAR-1/MacOS/TeamTalk5/Library/TeamTalk_DLL
 /usr/local/include
 /Users/bdr/tt5dist/toolchain/macos/ace/include
 /Users/bdr/tt5dist/toolchain/macos/openssl/include

/usr/local/include/openssl will therefore be included instead of
/Users/bdr/tt5dist/toolchain/macos/openssl/include

Author: bear101

Library/TeamTalkLib/CMakeLists.txt

@@ -56,6 +56,11 @@ if (${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
     message(WARNING "No architecture specified. Defaulting to ${CMAKE_OSX_ARCHITECTURES}")
   endif()
 
+  # Prevent clang from including /usr/local (where Homebrew installs
+  # its header files and libraries)
+  list (APPEND COMPILE_FLAGS -Xclang -nostdsysteminc)
+  set (LINK_LIBS -Xclang -nostdsysteminc)
+
 elseif (${CMAKE_SYSTEM_NAME} MATCHES "iOS")
 
   if (NOT CMAKE_OSX_DEPLOYMENT_TARGET)