[lua] Fix library type and usage

[JackBoosY]

Since the library generated by lua's feature cpp uses the same source as the c version of the lua library, using SET_SOURCE_FILES_PROPERTIES(PROPERTIES LANGUAGE CXX) anywhere in cmake will cause the c library to also be built as the cpp version.
So I made a copy of the source code and just declared them as CXX to fix this.
Also export the INTERFACE_INCLUDE_DIRECTORIES for usage.

Fixes #24413

Already tested build rmlui with / without lua cpp feature.

[BillyONeal]

Sorry, I should have clarified: the thing I want us to try is to have different directories with CMakeLists.txts in them, without any copying of source files. (If I have time today I'm going to try this...)

[JackBoosY]

According to the official doc:

New in version 3.18: By default, source file properties are only visible to targets added in the same directory (CMakeLists.txt). Visibility can be set in other directory scopes using one or both of the following options:

So we can make a new CMakeLists.txt, add the cpp code to it to avoid this.

[dg0yt]

• IMO the same list variables should be used for main directory and for C++ subdirectory. Avoid redundancy. Of course it needs absolute paths.
• Did you consider moving to file(GLOB ...) to collect the headers and sources?
• I see the install commands using absolute destination paths. In general, the preferred form is relative paths, and it is shorter. (Not so relevant for vcpkg but enough to catch my eye.)

[BillyONeal]

This addresses my concern of multiple copies of sources confusing debuggers, thanks!

I would like to see @dg0yt's comments addressed though.

[JackBoosY]

• IMO the same list variables should be used for main directory and for C++ subdirectory. Avoid redundancy. Of course it needs absolute paths.

Such as the headers and sources?

• Did you consider moving to file(GLOB ...) to collect the headers and sources?

No, lua provides more files than listed. We should only choose some of them.

• I see the install commands using absolute destination paths. In general, the preferred form is relative paths, and it is shorter. (Not so relevant for vcpkg but enough to catch my eye.)

I will make some update for them.

[BillyONeal]

Thanks for your fixes!