Add header-only Meson support (#955)

* Add header-only Meson support
This allows users to call `dependency('httplib')` and have the include
directory automatically configured

* Rename `httplib` to `cpp-httplib`

meson.build

@@ -0,0 +1,7 @@
+project('cpp-httplib', 'cpp', license: 'MIT')
+
+cpp_httplib_dep = declare_dependency(include_directories: include_directories('.'))
+
+if meson.version().version_compare('>=0.54.0')
+  meson.override_dependency('cpp-httplib', cpp_httplib_dep)
+endif