add --codegen-backends=foo,bar ./configure flag
add --codegen-backends=foo,bar ./configure flag

Unfortunately this requires a proper ./configure flag, as the codegen
backends config entry is a list, not a string (breaking --set).

graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust