Keep isfinite() but downgrade for platforms that lack it.
Centos 5 has it with _GNU_SOURCE
Older Solaris doesn't have it


git-svn-id: http://svn.osgeo.org/postgis/trunk@12854 b70326c6-7e19-0410-871a-916f4a2858ee
Make grep and sed swappable as well (Solaris again)


git-svn-id: http://svn.osgeo.org/postgis/trunk@12855 b70326c6-7e19-0410-871a-916f4a2858ee