MacOS build assumes libgdal via Homebrew

[metasim]

Evidenced by looking at the dylib loading rules:

$ curl -o gdal-warp.zip https://repo1.maven.org/maven2/com/azavea/geotrellis/gdal-warp-bindings/33.567d940/gdal-warp-bindings-33.567d940.jar
$ unzip gdal-warp.zip
$ otool -L resources/libgdalwarp_bindings.dylib

resources/libgdalwarp_bindings.dylib:
	libgdalwarp_bindings.dylib (compatibility version 0.0.0, current version 0.0.0)
	@@HOMEBREW_PREFIX@@/opt/gdal/lib/libgdal.20.dylib (compatibility version 26.0.0, current version 26.0.0)
	/usr/lib/libc++.1.dylib (compatibility version 1.0.0, current version 120.0.0)
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1213.0.0)

Build build should be updated to rely on the standard system lookup rules. Not sure how or what is injecting the @@HOMEBREW_PREFIX@@ bit.

The common use case is that users will have gdal via a conda or pip install.

Add'l context: https://gitter.im/geotrellis/geotrellis?at=5dcdbf95ea7d147cb37d438c

[jamesmcclain]

• https://gitter.im/locationtech/rasterframes?at=5dcdbfef6ba2347d2d943c5c
• https://stackoverflow.com/questions/33991581/install-name-tool-to-update-a-executable-to-search-for-dylib-in-mac-os-x
• https://stackoverflow.com/questions/24377639/can-i-change-homebrews-prefix/43326645#43326645