[build] Allow searching for external FreeType on MacOS

And fallback to using the builtin if the external can't be found.

Useful when there is an externally-provided FreeType library on MacOS, such as on conda-forge.

This is an upstream of https://github.com/conda-forge/root-feedstock/blob/df86316b299d8214d3181d76c31925696f076d5e/recipe/patches/0009-disable-builtin-freetype-macos.patch

Author: vepadulano

cmake/modules/SearchInstalledSoftware.cmake

@@ -105,7 +105,15 @@ endif()
 if(cocoa)
   if(APPLE)
     set(x11 OFF CACHE BOOL "Disabled because cocoa requested (${x11_description})" FORCE)
-    set(builtin_freetype ON CACHE BOOL "Enabled because needed for Cocoa graphics (${builtin_freetype_description})" FORCE)
+    # We allow using an external FreeType library
+    if(NOT builtin_freetype)
+      find_package(Freetype)
+      if(NOT FREETYPE_FOUND)
+        # FreeType is needed on MacOS, fallback to enabling builtin if it was not found
+        message(STATUS "FreeType not found. Switching on builtin_freetype option")
+        set(builtin_freetype ON CACHE BOOL "Enabled because needed for Cocoa graphics (${builtin_freetype_description})" FORCE)
+      endif()
+    endif()
   else()
     message(STATUS "Cocoa option can only be enabled on MacOSX platform")
     set(cocoa OFF CACHE BOOL "Disabled because only available on MacOSX (${cocoa_description})" FORCE)