README should suggest MacPorts installation of GraphicsMagick

[tensory]

While following https://github.com/cpp-io2d/P0267_RefImpl/blob/master/BUILDING.md on OSX 11.4 (Big Sur), I was not able to build io2D while GraphicsMagick was installed from Homebrew:

Found macOS, using CAIRO_XLIB.
-- Checking for module 'GraphicsMagick'
--   No package 'GraphicsMagick' found
CMake Error at /usr/local/Cellar/cmake/3.20.1/share/cmake/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find GraphicsMagick (missing: GRAPHICSMAGICK_INCLUDE_DIRS)

I got past this error by uninstalling the Homebrew copy of GraphicsMagick, and installing it from MacPorts instead:

brew uninstall graphicsmagick
sudo port install GraphicsMagick

[walterhudson]

Just ran into the same issue.

brew uninstall graphicsmagick
sudo port install GraphicsMagick

Fixed it.

[Frank1412]

same issue

[nagsconnect]

This helped, same issue