meson: Specify OEM extensions in an array

Prevent a proliferation of options by exploiting choice-constrained
arrays.

Change-Id: I5bb5c9bfd41d2854fafee189734abcf58b058d82
Signed-off-by: Andrew Jeffery <andrew@codeconstruct.com.au>
diff --git a/src/meson.build b/src/meson.build
index a4c8609..aa224f4 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -4,11 +4,11 @@
 subdir('requester')
 subdir('transport')
 
-if get_option('oem-ibm').allowed()
+if get_option('oem').contains('ibm')
     subdir('oem/ibm')
 endif
 
-if get_option('oem-meta').allowed()
+if get_option('oem').contains('meta')
     subdir('oem/meta')
 endif