gpsd: fix cross-compilation

Prior to this commit, builds of `pkgsCross.*.gpsd` were failing.

`gpsd` expects `CCVERSION` to be set, and fails if it is not set.
Scons does not guarantee that `CCVERSION` will be set, nor does it
specify under what circumstances it will or will not be set: "This may
or may not be set, depending on the specific C compiler being used."

  https://scons.org/doc/production/HTML/scons-man.html#cv-CCVERSION

Apparently cross-compilation triggers one of those unspecified
circumstances.  There are several bug reports to scons relating to
this:

  SCons/scons#1723
  https://github.com/SCons/scons/issues?q=is%3Aissue+ccversion+is%3Aclosed

`gpsd` does not use `CCVERSION` for any purpose other than printing a
log message at the start of the build:

  https://gitlab.com/gpsd/gpsd/-/commit/d0558636038e18d90eb886f668ff5004538f7839

This commit modifies the log message, replacing `env['CCVERSION']`
with `env['CC']`, since `CC` is always set when using nixpkgs'
standard builder.

With this commit, `pkgsCross.mips64el-linux-gnuabi64.gpsd` and
`pkgsCross.powernv.gpsd` build correctly on x86_64.  Prior to this
commit, they would fail with:

```
scons: Reading SConscript files ...
scons version: 4.1.0
scons is running under Python version: 3.10.5.final.0
gpsd version: 3.23.1
This system is: linux
KeyError: 'CCVERSION':
  File "/build/gpsd-3.23.1/SConstruct", line 69:
    SConscript('SConscript',
  File "/nix/store/0vmqv6f2s8bj2a50gk8g05jcb5scnifb-scons-4.1.0/lib/python3.10/site-packages/SCons/Script/SConscript.py", line 654:
    return method(*args, **kw)
  File "/nix/store/0vmqv6f2s8bj2a50gk8g05jcb5scnifb-scons-4.1.0/lib/python3.10/site-packages/SCons/Script/SConscript.py", line 591:
    return _SConscript(self.fs, *files, **subst_kw)
  File "/nix/store/0vmqv6f2s8bj2a50gk8g05jcb5scnifb-scons-4.1.0/lib/python3.10/site-packages/SCons/Script/SConscript.py", line 280:
    exec(compile(scriptdata, scriptname, 'exec'), call_stack[-1].globals)
  File "/build/gpsd-3.23.1/gpsd-3.23.1/SConscript", line 883:
    announce("cc is %s, version %s" % (env['CC'], env['CCVERSION']))
  File "/nix/store/0vmqv6f2s8bj2a50gk8g05jcb5scnifb-scons-4.1.0/lib/python3.10/site-packages/SCons/Environment.py", line 388:
    return self._dict[key]
```

pkgs/servers/gpsd/default.nix

@@ -81,6 +81,7 @@ stdenv.mkDerivation rec {
     patchShebangs .
     sed -e "s|systemd_dir = .*|systemd_dir = '$out/lib/systemd/system'|" -i SConscript
     export TAR=noop
+    substituteInPlace SConscript --replace "env['CCVERSION']" "env['CC']"
 
     sconsFlags+=" udevdir=$out/lib/udev"
     sconsFlags+=" python_libdir=$out/lib/${python3Packages.python.libPrefix}/site-packages"