Looks like it was built before ABI breakage, can you fix?
$ uname -rms -v
OpenBSD 6.0 GENERIC.MP#2094 amd64
$ cat /etc/pkg.conf
installpath = ftp.icm.edu.pl
$ doas pkg_add -iv gcc
Update candidates: quirks-2.234 -> quirks-2.234
quirks-2.234 signed on 2016-05-24T23:44:35Z
Can't install gcc-4.9.3p6 because of libraries
|library c.88.0 not found
| /usr/lib/libc.so.87.0 (system): bad major
Direct dependencies for gcc-4.9.3p6 resolve to gmp-5.0.2p3 mpfr-3.1.0.3p0
libmpc-0.9p1 gcc-libs-4.9.3p6
Full dependency tree is gcc-libs-4.9.3p6 gmp-5.0.2p3 mpfr-3.1.0.3p0 libmpc-0.9p1
