Fix building with clang on linux (take 2)

[thlorenz]

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.

NOTE: although these are the exact same changes applied to master previously I cannot currently build either on Mac or Linux, instead I get the following error:

npm ERR! Failed resolving git HEAD (git://github.com/rvagg/abstract-leveldown) fatal: ambiguous argument 'julian-empty-values': unknown revision or path not in the working tree.
npm ERR! Failed resolving git HEAD (git://github.com/rvagg/abstract-leveldown) Use '--' to separate paths from revisions, like this:
npm ERR! Failed resolving git HEAD (git://github.com/rvagg/abstract-leveldown) 'git <command> [<revision>...] -- [<file>...]'
npm ERR! Failed resolving git HEAD (git://github.com/rvagg/abstract-leveldown)
npm ERR! Error: Command failed: fatal: ambiguous argument 'julian-empty-values': unknown revision or path not in the working tree.
npm ERR! Use '--' to separate paths from revisions, like this:
npm ERR! 'git <command> [<revision>...] -- [<file>...]'
npm ERR!
npm ERR!     at ChildProcess.exithandler (child_process.js:648:15)
npm ERR!     at ChildProcess.emit (events.js:98:17)
npm ERR!     at maybeClose (child_process.js:756:16)
npm ERR!     at Socket.<anonymous> (child_process.js:969:11)
npm ERR!     at Socket.emit (events.js:95:17)
npm ERR!     at Pipe.close (net.js:465:12)
npm ERR! If you need help, you may report this *entire* log,
npm ERR! including the npm and node versions, at:
npm ERR!     <http://github.com/npm/npm/issues>

However once that is resolved, the changes should fix things as before.

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

> [email protected] 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.