[m4] Use AS_VAR_APPEND macro instead of "+="
[m4] Use AS_VAR_APPEND macro instead of "+="

Also get rid of unnecessary subshells.

Signed-off-by: Alexander Tsoy <e46fc836cca3acec03944314d1457c2ae6c68ef3@tsoy.me>

puleglot/dleyna-core,puleglot/dleyna-core,lferrandis/dleyna-core,01org/dleyna-core,lferrandis/dleyna-core,01org/dleyna-core