AC_PATH_XTRA

[Akim Demaille]

The doc says:

 - Macro: AC_PATH_XTRA
     An enhanced version of `AC_PATH_X'.  It adds the C compiler flags
     that X needs to output variable `X_CFLAGS', and the X linker flags
     to `X_LIBS'.  If X is not available, adds `-DX_DISPLAY_MISSING' to
     `X_CFLAGS'.

but it adds nothing to X_CFLAGS: it AC_DEFINE's this symbol.  So
should we adjust the doc, or the code?  Autoconf 2.13 already has this
inconsistency.  In practice X_CFLAGS contains only one `-I' at most.