I don't use vpopmail, but is seems like it couldn't find it.
> ...
> echo '#include "~vpopmail/include/config.h"' >vpopmail_config.h
> gcc -DHAVE_CONFIG_H -I. -I. -I. -I~vpopmail/include -g -O2 -Wall -I.. -I
> ./.. -c authvchkpw.c
> gcc -DHAVE_CONFIG_H -I. -I. -I. -I~vpopmail/include -g -O2 -Wall -I.. -I
> ./.. -c authvchkpwlib.c
> gcc -DHAVE_CONFIG_H -I. -I. -I. -I~vpopmail/include -g -O2 -Wall -I.. -I
> ./.. -c preauthvchkpw.c
> preauthvchkpw.c:19: vpopmail.h: No such file or directory
> preauthvchkpw.c:20: vauth.h: No such file or directory
> In file included from preauthvchkpw.c:21:
> vpopmail_config.h:1: ~vpopmail/include/config.h: No such file or directory
> make[1]: *** [preauthvchkpw.o] Error 1
> make[1]: Leaving directory
> `/export/home/macgyver/devel/qmail/sqwebmail-1.1.2/authlib'
> make: *** [all-recursive] Error 1
> ns3:~/devel/qmail/sqwebmail-1.1.2 42%
>
> ns3:/RAID/vpopmail/include 45% ls
> config.h vauth.h vpopmail.h
> vpopmail_config.h
> ns3:/RAID/vpopmail/include 46%
