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.
