- set macOS deployment target in CMake

Removed explicit version selection from Travis configuration
Bug in particular version of CMake requires CMAKE_OSX_SYSROOT to be set to existing SDK name for Xcode 8
https://cmake.org/pipermail/cmake/2012-November/052649.html

.travis.yml

@@ -14,13 +14,13 @@ matrix:
       os: osx
       osx_image: xcode8
       env:
-        - CMAKE_OPTIONS="-DCMAKE_BUILD_TYPE=Debug -DCMAKE_OSX_DEPLOYMENT_TARGET=10.9"
+        - CMAKE_OPTIONS="-DCMAKE_BUILD_TYPE=Debug -DCMAKE_OSX_SYSROOT=macosx10.12"
 
     - name: "Xcode 11.3"
       os: osx
       osx_image: xcode11.3
       env:
-        - CMAKE_OPTIONS="-DCMAKE_BUILD_TYPE=Release -DCMAKE_OSX_DEPLOYMENT_TARGET=10.9"
+        - CMAKE_OPTIONS="-DCMAKE_BUILD_TYPE=Release"
 
     - name: "Visual Studio 2017 32-bit"
       os: windows

CMakeLists.txt

@@ -288,6 +288,7 @@ else()
 
 
 	if( APPLE )
+		set(CMAKE_OSX_DEPLOYMENT_TARGET "10.9")
 		if( CMAKE_CXX_COMPILER_ID STREQUAL "Clang" )
 			# With standard Apple tools -stdlib=libc++ needs to be specified in order to get
 			# C++11 support using SDKs 10.7 and 10.8.