Define wxUSE_CAIRO in wx/chkconf.h to ensure that it's always defined.

wxUSE_CAIRO used to be defined only in wx/cairo.h but this header wasn't
included by src/common/dcgraph.cpp so the code there was compiled as if we
were not using Cairo even when we were (thanks g++ for the warning).

Define it in wx/chkconf.h (included from wx/defs.h, i.e. always) now to ensure
that not only this bug is fixed but also that it can't happen any more.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62730 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/chkconf.h

@@ -431,6 +431,18 @@
 #   endif
 #endif /* !defined(wxUSE_BUTTON) */
 
+/*
+    This one is special, it's only used under wxGTK currently so don't require
+    pre-defining it.
+ */
+#ifndef wxUSE_CAIRO
+#   ifdef __WXGTK210__
+#        define wxUSE_CAIRO 1
+#    else
+#        define wxUSE_CAIRO 0
+#    endif
+#endif
+
 #ifndef wxUSE_CALENDARCTRL
 #   ifdef wxABORT_ON_CONFIG_ERROR
 #       error "wxUSE_CALENDARCTRL must be defined, please read comment near the top of this file."