Building pkgtools/pkg_install fails with lang/gcc8

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

With PKGSRC_COMPILER=gcc, gcc8 from lang/gcc8, building pkgtools/pkg_install 
fails because compiling libfetch yields

In file included from /var/work/pkgsrc/pkgtools/pkg_install/work/libnbcompat/nbcompat/stdlib.h:36,
                 from /var/work/pkgsrc/pkgtools/pkg_install/work/libnbcompat/nbcompat.h:60,
                 from common.c:37:
/usr/pkg/gcc8/lib/gcc/x86_64--netbsd/8.4.0/include-fixed/stdlib.h:115:1: error:ignoring attribute 'pure' in declaration of a built-in function 'abs' because it conflicts with attribute 'const' [-Werror=attributes]
 __pure  int abs(int);
 ^~~~~~
<built-in>: note: previous declaration here
/usr/pkg/gcc8/lib/gcc/x86_64--netbsd/8.4.0/include-fixed/stdlib.h:132:3: error:ignoring attribute 'pure' in declaration of a built-in function 'labs' because it conflicts with attribute 'const' [-Werror=attributes]
   labs(long);
   ^~~~
<built-in>: note: previous declaration here
cc1: all warnings being treated as errors

Building net/libfetch gives the same warning, but doesn't error out.