Fix so custom --with-projdir= configure switch works again.

References #4279 for PostGIS 2.5

git-svn-id: http://svn.osgeo.org/postgis/branches/2.5@17119 b70326c6-7e19-0410-871a-916f4a2858ee
1 job for svn-2.5 in 22 minutes and 26 seconds
Status Job ID Name Coverage
  Test
passed #138774511
test

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