CMake: add all Homebrew packages to root search path.

Homebrew doesn't provide a merged (FHS) prefix and tools installed from
it cannot be expected to appear on PATH. Furthermore, XCode provides
some tools and headers (Flex, Bison) which must not be used if
a Homebrew alternative is installed.

CMakeLists.txt

@@ -29,6 +29,7 @@ include(YosysLinkTarget)
 include(YosysAbc)
 include(YosysAbcSubmodule)
 include(YosysVerific)
+include(UseHomebrew)
 
 # Build options.
 set(YOSYS_COMPILER_LAUNCHER "" CACHE STRING "Compiler launcher (ccache, sccache)")
@@ -162,6 +163,11 @@ if (MINGW AND CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSIO
 endif()
 
 # Required dependencies.
+if (APPLE)
+	# In practice, we can't expect paths to Homebrew packages to be configured.
+	use_homebrew()
+endif()
+
 find_package(FLEX)
 set_package_properties(FLEX PROPERTIES
 	URL "https://github.com/westes/flex"

cmake/UseHomebrew.cmake

@@ -0,0 +1,22 @@
+# Syntax:
+#
+# 	use_homebrew([ROOT <root>])
+#
+# Includes all packages installed in `<root>` (`/opt/homebrew/Cellar` if not specified)
+# in `CMAKE_FIND_ROOT_PATH`.
+#
+function(use_homebrew)
+	cmake_parse_arguments(PARSE_ARGV 0 arg "" "ROOT" "")
+	if (NOT arg_ROOT)
+		set(arg_ROOT /opt/homebrew/Cellar)
+	endif()
+
+	file(GLOB package_roots ${arg_ROOT}/*/*) # e.g. `/opt/homebrew/Cellar/bison/3.8.2/`
+	foreach (package_root ${package_roots})
+		if (IS_DIRECTORY ${package_root})
+			list(APPEND CMAKE_FIND_ROOT_PATH ${package_root})
+		endif()
+	endforeach()
+
+	return(PROPAGATE CMAKE_FIND_ROOT_PATH)
+endfunction()