Functions to avoid in library code

[richfitz]

* checking compiled code ... WARNING
File ‘jqr/libs/jqr.so’:
  Found ‘___stderrp’, possibly from ‘stderr’ (C)
    Objects: ‘execute.o’, ‘jv_alloc.o’, ‘jv_print.o’, ‘lexer.o’,
      ‘locfile.o’
  Found ‘___stdoutp’, possibly from ‘stdout’ (C)
    Objects: ‘jv_print.o’, ‘lexer.o’
  Found ‘_abort’, possibly from ‘abort’ (C)
    Object: ‘jv_alloc.o’
  Found ‘_exit’, possibly from ‘exit’ (C)
    Objects: ‘jq_test.o’, ‘lexer.o’
  Found ‘_printf’, possibly from ‘printf’ (C)
    Objects: ‘bytecode.o’, ‘execute.o’, ‘jq_test.o’
  Found ‘_putchar’, possibly from ‘putchar’ (C)
    Objects: ‘bytecode.o’, ‘execute.o’, ‘jq_test.o’
  Found ‘_puts’, possibly from ‘printf’ (C), ‘puts’ (C)
    Objects: ‘bytecode.o’, ‘jq_test.o’
  Found ‘_rand’, possibly from ‘rand’ (C)
    Object: ‘jq_test.o’

Compiled code should not call entry points which might terminate R nor
write to stdout/stderr instead of to the console, nor the system RNG.

[richfitz]

Interestingly this only happens when building the sources within src (which I'm trying at the moment) rather than building as a library and then linking (which is what is on the travis log at the moment).