Add clang-format support

# clang_format([DEPENDENCY] [GLOBAL] [REGEX "..."] TARGETS "..." ["..."])
Adds a clang-format call target for each of the specified targets, which optionally filters files using a special regex (defaults to .c, .cpp, .h, .hpp), with an optional global target, and an optional dependency addition to ensure that formatting is applied before building any of the targets.

* DEPENDENCY: Add a dependency to each of the targets onto the clang-format target, to ensure that formatting is applied at build time.
* GLOBAL: Create a global target and add the clang-format target as a dependency to it.
* REGEX "...": Filter file by a custom regular expression instead of the default filter. Must be a regular expression.
* TARGETS "...": Specifies the targets that clang-format is used for.

Clang.cmake

@@ -0,0 +1,65 @@
+function(clang_format)
+	cmake_parse_arguments(
+		PARSE_ARGV 0
+		_CLANG_FORMAT
+		"DEPENDENCY;GLOBAL"
+		"REGEX"
+		"TARGETS"
+	)
+
+	find_program(CLANG_FORMAT_BIN "clang-format" DOC "Path (or name) of the clang-format binary")
+	if(NOT CLANG_FORMAT_BIN)
+		message(WARNING "Clang: Could not find clang-format at path '${CLANG_FORMAT_BIN}'. Disabling clang-format...")
+		return()
+	endif()
+
+	if(NOT _CLANG_FORMAT_FILTER)
+		set(_CLANG_FORMAT_FILTER "\.(h|hpp|c|cpp)$")
+	endif()
+
+	foreach(_target ${_CLANG_FORMAT_TARGETS})
+#		get_target_property(target_name ${_target} NAME)
+
+		get_target_property(target_sources_rel ${_target} SOURCES)
+		set(target_sources "")
+		foreach(source_relative ${target_sources_rel})
+			get_filename_component(source_absolute ${source_relative} ABSOLUTE)
+			list(APPEND target_sources ${source_absolute})
+		endforeach()
+		list(FILTER target_sources INCLUDE REGEX "${_CLANG_FORMAT_FILTER}")
+		unset(target_sources_rel)
+		
+		get_target_property(target_source_dir_rel ${_target} SOURCE_DIR)
+		get_filename_component(target_source_dir ${target_source_dir_rel} ABSOLUTE)
+		unset(target_source_dir_rel)
+
+		add_custom_target(${_target}_CLANG-FORMAT
+			COMMAND
+				${CLANG_FORMAT_BIN}
+				-style=file
+				-i
+				${target_sources}
+			COMMENT
+				"clang-format: Formatting ${_target}..."
+			WORKING_DIRECTORY
+				${target_source_dir_rel}
+		)
+
+		if(_CLANG_FORMAT_DEPENDENCY)
+			add_dependencies(${_target} ${_target}_CLANG-FORMAT)
+		endif()
+
+		if(_CLANG_FORMAT_GLOBAL)
+			if(TARGET CLANG-FORMAT)
+				add_dependencies(CLANG-FORMAT ${_target}_CLANG-FORMAT)
+			else()
+				add_custom_target(CLANG-FORMAT
+					DEPENDS
+						${_target}_CLANG-FORMAT
+					COMMENT
+						"clang-format: Formatting..."
+				)
+			endif()
+		endif()
+	endforeach()
+endfunction()