Add ".s" to source extensions.

It uses ".s" as the source for assembler on some toolchains including GCC.
Find c++ files in tools/.

Some tools may be written in C++.
Do not export include/ dirs in mrbgems to core build.

In the current behavior, include/ dirs in mrbgems are set to
core's compiler.include_paths.
And they are never set to mrbgem's include_paths.
It may cause some dangerous issues because it can change
mruby core's macros by mrbgems.

After this fix, include/ in a gem is used in the gem itself only.
Merge pull request #2379 from monaka/pr-do-not-export-include-dir-in-gems-to-core

Do not export include/ dirs in mrbgems to core build.
Merge pull request #2378 from monaka/pr-add-tools-dir-to-cxx-files-check-oaths

Find c++ files in tools/.
Merge pull request #2377 from monaka/pr-add-dot-s-extension

Add ".s" to source extensions.
Add include_paths in dependency gems.
Add more info about "include_paths and GEM dependency".
Merge pull request #2381 from monaka/pr-add-include_paths-in-dependency-gems

Append include paths in dependency gems
Merge branch 'master' of github.com:mruby/mruby
Build sources in {GEM_NAME}/tools/{BIN_NAME}/ with the configuration for {GEM_NAME}.