fix some codegen bugs: NIM_BOOL, NIM_STATIC_ASSERT, --passc:-std=... (etc) (#13798)

* fix cgen bool D20200328T203812
* --passc:std=c++17 (etc) now works instead of silently ignored
* document caveats for NIM_NIL

tests/vm/tevalffi.nim

@@ -1,28 +0,0 @@
-discard """
-  joinable: false
-"""
-
-import std/[strformat,os,osproc]
-
-proc main() =
-  const nim = getCurrentCompilerExe()
-  const file = currentSourcePath().parentDir / "mevalffi.nim"
-  # strangely, --hint:cc:off was needed
-  let cmd = fmt"{nim} c -f --experimental:compiletimeFFI --hints:off --hint:cc:off {file}"
-  let (output, exitCode) = execCmdEx(cmd)
-  let expected = """
-hello world stderr
-hi stderr
-foo
-foo:100
-foo:101
-foo:102:103
-foo:102:103:104
-foo:0.03:asdf:103:105
-ret={s1:foobar s2:foobar age:25 pi:3.14}
-"""
-  doAssert output == expected, output
-  doAssert exitCode == 0
-
-when defined(nimHasLibFFIEnabled):
-  main()