generalise the linker flags used to export symbols by setting them on
a per-OS basis.
> many packages force -Wl,-export-dynamic which is not portable outside GNU ld
> and cause problems e.g. on Solaris. some of these packages use if
> conditionals either only for NetBSD or except SunOS, but the state is not
> coherent and it may complicate later when support for new OS is added to
> pkgsrc (e.g. ongoing work on HP-UX support).
>
> jlam proposed the following framework in discussion on tech-pkg:
>
> http://mail-index.netbsd.org/tech-pkg/2002/06/21/0009.html
>
> now, ${EXPORT_SYMBOLS_LDFLAGS} is used instead of directly defining
> -Wl,-export-dynamic which is set in appropriate defs.*.mk to reasonable
> values. packages should be converted to this framework by:
>
> 1) replacing LDFLAGS+= -Wl,-export-dynamic and LIBS+= -export-dynamic with:
>
> LDFLAGS+= ${EXPORT_SYMBOLS_LDFLAGS}
>
> 2) for use in patchfiles, add this variable to MAKE_ENV if needed:
>
> MAKE_ENV+= EXPORT_SYMBOLS_LDFLAGS=${EXPORT_SYMBOLS_LDFLAGS}
>
> 3) replace occurances of -Wl,-export-dynamic and -export-dynamic in patch
> files with:
>
> $(EXPORT_SYMBOLS_LDFLAGS)
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