meson: Add _BSD_SOURCE and _DEFAULT_SOURCE

To avoid warnings about string functions like strdup which are otherwise
undefined, but succeed at linking anyway when the C standard is c99.

meson.build

@@ -6,6 +6,11 @@ project('pkgconf', 'c',
 
 cc = meson.get_compiler('c')
 
+add_project_arguments(
+  '-D_BSD_SOURCE',
+  '-D_DEFAULT_SOURCE',
+  language : 'c',
+)
 
 cdata = configuration_data()