meson: add tests (#1044)

This integrates the "main" test suite (test/test.cc) in Meson.

This allows to run the tests in the CI with the Meson-built version of
the library to ensure that nothing breaks unexpectedly.

It also simplifies life of downstream packagers, that do not have to
write a custom build script to split the library and run tests but can
instead just let Meson do that for them.

meson.build

@@ -7,6 +7,7 @@ project(
   'cpp',
   license: 'MIT',
   default_options: [
+    'cpp_std=c++11',
     'buildtype=release',
     'b_ndebug=if-release',
     'b_lto=true',
@@ -71,7 +72,7 @@ if brotli_found_all
   args += '-DCPPHTTPLIB_BROTLI_SUPPORT'
 endif
 
-cpp_httplib_dep = dependency('', required : false)
+cpp_httplib_dep = dependency('', required: false)
 
 if get_option('cpp-httplib_compile')
   httplib_ch = custom_target(
@@ -106,3 +107,7 @@ endif
 if meson.version().version_compare('>=0.54.0')
   meson.override_dependency('cpp-httplib', cpp_httplib_dep)
 endif
+
+if get_option('cpp-httplib_test')
+  subdir('test')
+endif