cmake: Check for GNU compiler, not Linux, to set -D_GNU_SOURCE

The Debian hurd-i386 [build] failed (partly) due to -D_GNU_SOURCE not be
defined:

    [215/286] /usr/bin/cc -DINCLUDE_GENERATED_DECLARATIONS -Iconfig -I../src -Isrc/nvim/auto -Iinclude -I/usr/include/luajit-2.1 -g -O2 -fdebug-prefix-map=/<<PKGBUILDDIR>>=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -DDISABLE_LOG -Wdate-time -D_FORTIFY_SOURCE=2 -Wconversion -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -O2 -g  -DMIN_LOG_LEVEL=3 -Og -g   -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fdiagnostics-color=auto -Wno-array-bounds -MD -MT src/nvim/CMakeFiles/nvim.dir/os/pty_process_unix.c.o -MF src/nvim/CMakeFiles/nvim.dir/os/pty_process_unix.c.o.d -o src/nvim/CMakeFiles/nvim.dir/os/pty_process_unix.c.o   -c ../src/nvim/os/pty_process_unix.c
    ../src/nvim/os/pty_process_unix.c: In function 'pty_process_tty_name':
    ../src/nvim/os/pty_process_unix.c:121:10: warning: implicit declaration of function 'ptsname'; did you mean 'ttyname'? [-Wimplicit-function-declaration]
       return ptsname(ptyproc->tty_fd);

Hurd is obviously not Linux, but it is using a GNU compiler and glibc so
it needs -D_GNU_SOURCE for the ptsname() definition to be visible.

[build]: https://buildd.debian.org/status/fetch.php?pkg=neovim&arch=hurd-i386&ver=0.3.0-2&stamp=1528981349&raw=0

0 files changed, 0 insertions, 0 deletions