cmake: introduce MakeLuaPath.cmake helper

While extending tests it is often required to append additional
path where Lua or Lua C auxiliary modules are located to LUA_PATH or
LUA_CPATH environment variables. Due to insane semicolon interpolation
in CMake strings (that converts such string to a list as a result),
we need to escape semicolon in LUA_PATH/LUA_CPATH strings while
building the resulting value.

The patch introduce MakeLuaPath.cmake module to make LUA_PATH and
LUA_CPATH definition convenient with <make_lua_path> helper. This
function takes all paths given as a variable list argument,
joins them in a reverse order by a semicolon and yields the resulting
string to a specified CMake variable.

Needed for the following commit.

Author: ligurio

cmake/MakeLuaPath.cmake

@@ -0,0 +1,53 @@
+# make_lua_path provides a convenient way to define LUA_PATH and
+# LUA_CPATH variables.
+#
+# Example usage:
+#
+#   make_lua_path(LUA_PATH
+#     PATH
+#       ./?.lua
+#       ${CMAKE_BINARY_DIR}/?.lua
+#       ${CMAKE_CURRENT_SOURCE_DIR}/?.lua
+#   )
+#
+# This will give you the string:
+#    "./?.lua;${CMAKE_BINARY_DIR}/?.lua;${CMAKE_CURRENT_SOURCE_DIR}/?.lua;;"
+
+function(make_lua_path path)
+  set(prefix ARG)
+  set(noValues)
+  set(singleValues)
+  set(multiValues PATHS)
+
+  # FIXME: if we update to CMake >= 3.5, can remove this line.
+  include(CMakeParseArguments)
+  cmake_parse_arguments(${prefix}
+                        "${noValues}"
+                        "${singleValues}"
+                        "${multiValues}"
+                        ${ARGN})
+
+  set(_MAKE_LUA_PATH_RESULT "")
+
+  foreach(inc ${ARG_PATHS})
+    # XXX: If one joins two strings with the semicolon, the value
+    # automatically becomes a list. I found a single working
+    # solution to make result variable be a string via "escaping"
+    # the semicolon right in string interpolation.
+    set(_MAKE_LUA_PATH_RESULT "${_MAKE_LUA_PATH_RESULT}${inc}\;")
+  endforeach()
+
+  if("${_MAKE_LUA_PATH_RESULT}" STREQUAL "")
+    message(FATAL_ERROR "No paths are given to <make_lua_path> helper.")
+  endif()
+
+  # XXX: This is the sentinel semicolon having special meaning
+  # for LUA_PATH and LUA_CPATH variables. For more info, see the
+  # link below:
+  # https://www.lua.org/manual/5.1/manual.html#pdf-LUA_PATH
+  set(${path} "${_MAKE_LUA_PATH_RESULT}\;" PARENT_SCOPE)
+  # XXX: Unset the internal variable to not spoil CMake cache.
+  # Study the case in CheckIPOSupported.cmake, that affected this
+  # module: https://gitlab.kitware.com/cmake/cmake/-/commit/4b82977
+  unset(_MAKE_LUA_PATH_RESULT)
+endfunction()