Added meson file, to declare a dependency. (#232)

diff --git a/meson.build b/meson.build
new file mode 100644
index 0000000..704cda6
--- /dev/null
+++ b/meson.build
@@ -0,0 +1,3 @@
+project('doctest', ['cpp'], version: '2.3.1', meson_version:'>=0.50')
+doctest_dep = declare_dependency(include_directories: include_directories('doctest'))
+