Add meson subproject support

[pavmk]

This PR add meson subproject support.
F.ex., place next wrap in your subprojects directory:

[wrap-git]
revision = head
url = https://github.com/smarttowel/cpp-httplib.git

In your meson.build add next lines:

httplib_proj = subproject('httplib')
httplib_dep = httplib_proj.get_variable('httplib_dep')

[yhirose]

@smarttowel, thank you for your pull request. But I am personally not interested in any build system. In fact, there used to be a CMakeLists.txt in this project, but I removed it. 8674555
Anyway, I appreciated your effort and interest in this project.