Define wxUSE_UNICODE on compiler command line in Unix builds too.

Under Windows we already define _UNICODE which gets picked up and translated
to wxUSE_UNICODE by wx/platform.h, but under Unix we didn't do it until now.
Do it both for consistency and to fix the build breakage since r75520 which
moved the tests for wxUSE_UNICODE being defined before wx/setup.h.

This seems like the simplest solution because otherwise there doesn't seem to
be any single header order which would work under both Windows and Unix: under
Windows we need to include wx/compiler.h, and hence _mingw.h included from it,
before wx/setup.h, which implies that wxUSE_UNICODE must be set before doing
it, but under Unix wxUSE_UNICODE was only defined in wx/setup.h and so
couldn't be tested before including wx/compiler.h.

And just the explanation above should convincingly show that defining
wxUSE_UNICODE in CPPFLAGS makes things simpler.

Closes #15805.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75536 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

configure

@@ -35598,6 +35598,8 @@ if test "$wxUSE_FONTMAP" = "yes" ; then
 fi
 
 if test "$wxUSE_UNICODE" = "yes" ; then
+  CPPFLAGS="-DwxUSE_UNICODE=1 $CPPFLAGS"
+
   $as_echo "#define wxUSE_UNICODE 1" >>confdefs.h
 
 

configure.in

@@ -6397,6 +6397,8 @@ if test "$wxUSE_FONTMAP" = "yes" ; then
 fi
 
 if test "$wxUSE_UNICODE" = "yes" ; then
+  CPPFLAGS="-DwxUSE_UNICODE=1 $CPPFLAGS"
+
   AC_DEFINE(wxUSE_UNICODE)
 
   if test "$USE_WIN32" != 1; then