Move the Darwin binary dir next to Darwin

This allows rebuilding Darwin only with:

    cmake --build $INSTALLER_BASE/Darwin.build

Closes issue #16.

CMakeLists.txt

@@ -49,6 +49,7 @@ ExternalProject_Add(Darwin
     GIT_TAG             origin/master
     SOURCE_DIR          "${CMAKE_SOURCE_DIR}/Darwin"
     CMAKE_ARGS          -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR> -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
+    BINARY_DIR          "${CMAKE_SOURCE_DIR}/Darwin.build"
     INSTALL_DIR         "${CMAKE_INSTALL_PREFIX}"
     TEST_AFTER_INSTALL  ON
 )