autoconf: Move export_dynamic determination to configure
Previously export_dynamic was set in src/makefiles/Makefile.$port. For solaris
this required exporting with_gnu_ld. The determination of with_gnu_ld would be
nontrivial to copy for meson PGXS compatibility. It's also nice to delete
libtool.m4.
This uses -Wl,--export-dynamic on all platforms, previously all platforms but
FreeBSD used -Wl,-E. The likelihood of a name conflict seems lower with the
longer spelling.
Discussion: https://postgr.es/m/
20221005200710.luvw5evhwf6clig6@awork3.anarazel.de