Fix building with clang on linux

Suppressing `-fno-tree-vrp` flag in order to allow building with clang.

The build on Mac (also with clang) was fine before that, this fix allows building with clang on Linux as well.

#### clang version on Linux

```
➝  clang --version
clang version 3.4.2 (tags/RELEASE_34/dot2-final)
Target: x86_64-unknown-linux-gnu
Thread model: posix
```

#### clang version on Mac

As noted above this worked fine already.

```
➝  clang --version
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
Target: x86_64-apple-darwin13.3.0
Thread model: posix
```

#### Before this change building with clang on Linux

```
> leveldown@0.10.2 install /home/vagrant/dev/js/_forks/leveldown
> node-gyp rebuild

make: Entering directory '/home/vagrant/dev/js/_forks/leveldown/build'
  CXX(target) Release/obj.target/leveldb/deps/leveldb/leveldb-1.14.0/db/builder.o
clang: error: unknown argument: '-fno-tree-vrp'
deps/leveldb/leveldb.target.mk:132: recipe for target 'Release/obj.target/leveldb/deps/leveldb/leveldb-1.14.0/db/builder.o' failed
make: *** [Release/obj.target/leveldb/deps/leveldb/leveldb-1.14.0/db/builder.o] Error 1
make: Leaving directory '/home/vagrant/dev/js/_forks/leveldown/build'
```

#### After this change building with clang on Linux

It builds successfully even though it prints a the below warning multiple times:

```
warning: unknown warning option '-Wno-unused-but-set-variable'; did you mean '-Wno-unused-const-variable'? [-Wunknown-warning-option]
```

However this warning can be ignored and the important aspect is that it now builds successfully.