[PostGIS] #6112: README.postgis does not document gmp requirement

[PostGIS]

#6112: README.postgis does not document gmp requirement
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 Reporter:  gdt      |      Owner:  pramsey
     Type:  defect   |     Status:  new
 Priority:  low      |  Milestone:  PostGIS 3.7.0
Component:  postgis  |    Version:  master
 Keywords:           |
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 Building 3.7.0beta2 under pkgsrc, cmake objects to gmp not being present
 and the build fails.  Reading README.postgis, there's no mention of gmp
 being required, and why it is needed.
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Ticket URL: <https://trac.osgeo.org/postgis/ticket/6112>
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