Only include tests in subproject if explicitly requested (#41)

Author: awvwgk

meson.build

@@ -77,4 +77,6 @@ if install
 endif
 
 # add the testsuite
-subdir('test')
+if get_option('testing').auto() ? not meson.is_subproject() : get_option('testing').enabled()
+  subdir('test')
+endif

meson_options.txt

@@ -32,3 +32,10 @@ option(
   value: 'auto',
   description: 'Support quadruple precision',
 )
+
+option(
+  'testing',
+  type: 'feature',
+  value: 'auto',
+  description: 'Enable testing of test-drive library',
+)