Define __WXOSX__ and __WXMAC__ on compiler command line.

Define __WXOSX__ and __WXMAC__ on compiler command line.

Although __WXMAC__ and __WXMAC__ were already defined in wx/platform.h if
__WXOSX_XXX__ was defined, they couldn't be used for checks done before
including this header, which was surprising and, in case of __WXMAC__,
backwards incompatible. Define them now on the compiler command line to ensure
that they are always defined.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62353 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775