in addition to 1.42's process,

in the following make files,

third-party/libnatpmp/Makefile
third-party/miniupnp/Makefile
libtransmission/Makefile
daemon/Makefile
cli/Makefile

remove
-Wextra
-Winit-self
-Wmissing-format-attribute

Also in those Makefiles,remove
-std=gnu99
from the CFLAGS = line

At this point, make will complain about :
libtransmission/net.c:56: `IN6ADDR_ANY_INIT' undeclared here (not in a function

For fixing it, the best I know so far is :
in libtransmission/net.h
add:
#if defined(HAIKU)
#include <resolv.h>
#endif

However that'll generate some more errors:
/boot/develop/headers/posix/resolv.h:160: field `nsaddr_list' has incomplete type
/boot/develop/headers/posix/resolv.h:170: field `addr' has incomplete type
/boot/develop/headers/posix/resolv.h:194: field `sin' has incomplete type
transmission-1.42+-svn7652/libtransmission/bandwidth.c: In function `tr_bandwidthAllocate':
transmission-1.42+-svn7652/libtransmission/bandwidth.c:228: warning: unknown conversion type character `z' in format
transmission-1.42+-svn7652/libtransmission/bandwidth.c:228: warning: too many arguments for format
make[2]: * [bandwidth.o] Error 1
make[2]: Leaving directory `transmission-1.42+-svn7652/libtransmission'