meson: remove OS definitions from config_targetos

CONFIG_DARWIN, CONFIG_LINUX and CONFIG_BSD are used in some rules, but
only CONFIG_LINUX has substantial use.  Convert them all to if...endif.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

scsi/meson.build

@@ -1,4 +1,6 @@
 block_ss.add(files('utils.c'))
-block_ss.add(when: 'CONFIG_LINUX',
-             if_true: files('pr-manager.c', 'pr-manager-helper.c'),
-             if_false: files('pr-manager-stub.c'))
+if targetos == 'linux'
+  block_ss.add(files('pr-manager.c', 'pr-manager-helper.c'))
+else
+  block_ss.add(files('pr-manager-stub.c'))
+endif