build: avoid check-symfile on non-Linux

build: avoid check-symfile on non-Linux

I tested both OpenBSD and cygwin; both failed 'make check' with:

  GEN    check-symfile
Can't return outside a subroutine at ./check-symfile.pl line 13.

Perl requires 'exit 77' instead of 'return 77' in that context,
but even with that tweak, the build still fails, since the exit
code of 77 is only special to explicit TESTS=foo listings, and
not to make-only dependency rules where we are not going through
automake's test framework.

* src/check-symfile.pl: Kill bogus platform check...
* src/Makefile.am (check-symfile): ...and replace with an automake
conditional.