opencv: modify CMake config to avoid Cellar paths

[cho-m]

Homebrew-specific workaround due to our symlink directory structure.

REALPATH would usually be the better option since it avoids other symlink issues (e.g. if you only symlink CMake files and then try to use those, they can end up using incorrect paths if CMAKE_CURRENT_LIST_DIR isn't resolved first).

In Homebrew, for standard layout, this isn't usually not an issue as we symlink all parts of keg. It mainly becomes an issue if using libexec or other non-linked directories.

---

Specifically to avoid having to patch these up afterwards like in visp:

homebrew-core/Formula/v/visp.rb

Lines 133 to 144 in dc51e30

	# Replace generated references to OpenCV's Cellar path
	opencv = Formula["opencv"]
	opencv_references = Dir[
	"CMakeCache.txt",
	"CMakeFiles/Export/lib/cmake/visp/VISPModules.cmake",
	"VISPConfig.cmake",
	"VISPGenerateConfigScript.info.cmake",
	"VISPModules.cmake",
	"modules/**/flags.make",
	"unix-install/VISPConfig.cmake",
	]
	inreplace opencv_references, opencv.prefix.realpath, opencv.opt_prefix

homebrew-core/Formula/v/visp.rb

Lines 148 to 150 in dc51e30

	# Make sure software built against visp don't reference opencv's cellar path either
	inreplace [lib/"pkgconfig/visp.pc", lib/"cmake/visp/VISPConfig.cmake", lib/"cmake/visp/VISPModules.cmake"],
	opencv.prefix.realpath, opencv.opt_prefix