pkg/52648: pkg_install does not build on Linux, breaks bootstrap

[bsiegert%gmail.com@localhost]

>Number:         52648
>Category:       pkg
>Synopsis:       pkg_install does not build on Linux, breaks bootstrap
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    pkg-manager
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Tue Oct 24 07:40:00 +0000 2017
>Originator:     Benny Siegert
>Release:        pkgsrc-current as of 2017-10-24
>Organization:
The NetBSD Foundation
>Environment:
Linux [redacted] 4.9.0-4-amd64 #1 SMP Debian 4.9.51-1 (2017-09-28) x86_64 GNU/Linux
>Description:
Bootstrapping current pkgsrc on Debian GNU/Linux fails because pkg_install fails to build. (Perhaps I am missing some headers, but which ones?)

--- gpgsig.o ---
gpgsig.c: In function 'detached_gpg_sign':
gpgsig.c:126:4: warning: ignoring return value of 'write', declared with attribute warn_unused_result [-Wunused-result]
    write(STDERR_FILENO, err_msg, sizeof(err_msg) - 1);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
gpgsig.c:136:4: warning: ignoring return value of 'write', declared with attribute warn_unused_result [-Wunused-result]
    write(STDERR_FILENO, err_msg, sizeof(err_msg) - 1);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
--- pkcs7.o ---
pkcs7.c: In function 'check_ca':
pkcs7.c:63:11: error: dereferencing pointer to incomplete type 'X509 {aka struct x509_st}'
  if ((cert->ex_flags & EXFLAG_KUSAGE) != 0 &&
           ^~
*** [pkcs7.o] Error code 1

bmake[1]: stopped in /home/bsiegert/pkgsrc/bootstrap/work/wrk/pkgtools/pkg_install/work/pkg_install-20170419/lib
1 error

>How-To-Repeat:
My command line was

./bootstrap --make-jobs=12 --unprivileged --prefix=$HOME/pkg --pkgdbdir=$HOME/pkg/pkgdb --varbase=$HOME/pkg/var
>Fix: