Thx to Uberlord I got past the last Asterisk build failure. Though I hit another issue in dns.c:

dns.o: In function `ast_search_dns':
dns.c:(.text+0x355): undefined reference to `__res_close'
collect2: ld returned 1 exit status
make: *** [asterisk] Error 1


Seems like the latest uclibc-0.9.29 doesn't provide res_close by default. So I had to enable UCLIBC_HAS_BSD_RES_CLOSE in the ebuild on bug #182094. At least Asterisk now compiles :)