Build script: Allow to change sbindir and mandir

I'm interested in packaging NTPsec for Archlinux. The problem is that the filesystem layout on Arch differs from the defaults of the waf buildscript:

/bin, /sbin, /usr/sbin are all merged into /usr/bin and there is no /usr/local. The latter is no problem as it can easily be changed using the --prefix=/usr option. The thing is, that there is no option to set --sbindir and thus ntpd gets installed into /usr/sbin. Another issue is that there is no way to set the directory for the man pages. Archlinux has them under /usr/share/man, but the waf script copies them into /usr/man. There is an --path-doc= option, but AFAICT this doesn't affect anything.

$ ./waf configure --prefix=/usr --bindir=/usr/bin --enable-crypto
...
$ ./waf build
...
$ ./waf install --destdir=../tmp
...
$ ls -lR ../tmp
total 0
drwxr-xr-x  5 wlhlm  staff  170 Jun  4 14:41 usr

../tmp/usr:
total 0
drwxr-xr-x  9 wlhlm  staff  306 Jun  4 14:41 bin
drwxr-xr-x  5 wlhlm  staff  170 Jun  4 14:41 man
drwxr-xr-x  3 wlhlm  staff  102 Jun  4 14:41 sbin

../tmp/usr/bin:
total 1952
-rwxr-xr-x  1 wlhlm  staff  137404 Jun  4 14:30 ntpdig
-rwxr-xr-x  1 wlhlm  staff   54972 Jun  4 14:30 ntpfrob
-rwxr-xr-x  1 wlhlm  staff   61696 Jun  4 14:30 ntpkeygen
-rwxr-xr-x  1 wlhlm  staff  204056 Jun  4 14:30 ntpq
-rwxr-xr-x  1 wlhlm  staff  197268 Jun  4 14:30 test_libntp
-rwxr-xr-x  1 wlhlm  staff  129744 Jun  4 14:30 test_ntpd
-rwxr-xr-x  1 wlhlm  staff  200036 Jun  4 14:30 test_ntpdig

../tmp/usr/man:
total 0
drwxr-xr-x  6 wlhlm  staff  204 Jun  4 14:41 man1
drwxr-xr-x  4 wlhlm  staff  136 Jun  4 14:41 man5
drwxr-xr-x  8 wlhlm  staff  272 Jun  4 14:41 man8

../tmp/usr/man/man1:
total 32
-rw-r--r--  1 wlhlm  staff  1500 Jun  4 14:37 ntpdig.1
-rw-r--r--  1 wlhlm  staff  1552 Jun  4 14:38 ntpq.1
-rw-r--r--  1 wlhlm  staff  2784 Jun  4 14:40 ntpsweep.1
-rw-r--r--  1 wlhlm  staff  1478 Jun  4 14:40 ntptrace.1

../tmp/usr/man/man5:
total 40
-rw-r--r--  1 wlhlm  staff  13297 Jun  4 14:37 ntp.conf.5
-rw-r--r--  1 wlhlm  staff   3153 Jun  4 14:37 ntp.keys.5

../tmp/usr/man/man8:
total 48
-rw-r--r--  1 wlhlm  staff  1521 Jun  4 14:37 ntpd.8
-rw-r--r--  1 wlhlm  staff  1476 Jun  4 14:38 ntpfrob.8
-rw-r--r--  1 wlhlm  staff  1488 Jun  4 14:39 ntpkeygen.8
-rw-r--r--  1 wlhlm  staff  1505 Jun  4 14:40 ntpleapfetch.8
-rw-r--r--  1 wlhlm  staff  1489 Jun  4 14:38 ntptime.8
-rw-r--r--  1 wlhlm  staff  1494 Jun  4 14:40 ntpwait.8

../tmp/usr/sbin:
total 912
-rwxr-xr-x  1 wlhlm  staff  464232 Jun  4 14:30 ntpd