internal: Add CONCAT macro
internal: Add CONCAT macro

Using the two-step idiom to force resolution of other macros, e.g.:

  #define bar BAR
  CONCAT_(foo, bar)     // foobar
  CONCAT(foo, bar)      // fooBAR

Signed-off-by: Tim Wiederhake <da93ee425e056cfcf1ddac9fc87851b25464abf4@redhat.com>
Reviewed-by: Martin Kletzander <e82f23e74d43760279ebcc505dac4ae70d5627d0@redhat.com>
Reviewed-by: Daniel P. Berrangé <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

crobinso/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,zippy2/libvirt,zippy2/libvirt,libvirt/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,jfehlig/libvirt,crobinso/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,jfehlig/libvirt,olafhering/libvirt,crobinso/libvirt,zippy2/libvirt,zippy2/libvirt