The attributes in question are:
- nonnull: specify whether a function argument cannot/may not be null
- returns_nonnull: specify whether a function will not return a null
pointer (example: xmalloc can't return null, so it should be annotated as
such). Only available from gcc 4.9 onwards.
Currently these attributes are only supported by gcc.
The attribute was removed in commit c047507 in the clang repository as it
was never properly implemented anyway. This fixes compiling with clang 3.5.
Fixes issue #429
The SHELL_* defines are the bitflags that can be passed to `mch_call_shell`.
The enum is defined in 'os/shell.h', where all shell-related functions will
eventually be defined.
I'm debugging OOM behavior using http://www.nongnu.org/failmalloc/ on Linux.
gdb environment:
```
set env LD_PRELOAD=libfailmalloc.so
set env FAILMALLOC_SPACE=0xF00000
```
SEGV was happening like this:
```
Starting program: /home/felipe/code/neovim/build/bin/nvim
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Vim: Error: Out of memory.
Program received signal SIGSEGV, Segmentation fault.
0x00000000004d3719 in getout (exitval=1) at
/home/felipe/code/neovim/src/main.c:836
836 if (*p_viminfo != NUL)
(gdb)
```
After the fix it works as expected:
```
(gdb) set environment LD_PRELOAD=libfailmalloc.so
(gdb) set environment FAILMALLOC_SPACE=0xF00000
(gdb) r
Starting program: /home/felipe/code/neovim/build/bin/nvim
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Vim: Error: Out of memory.
Vim: Finished.
[Inferior 1 (process 21505) exited with code 01]
(gdb)
```
It appears the llvm.org/apt/ repository isn't always reliable. So let's
use the release tarball instead. Also, make using 3.4 conditional, so
we can use the clang 3.3 if things still manage to go awry in the
future. Note: using 3.3 means that we won't get leak detection.
I left the logic for using llvm.org/apt/, just in case we want try using
it again sometime.
