Use clang++ on mac (if present).
Enable march detection with clang.
Add a meta-optimizer for GpuConv
Add an additional meta-optimizer entry that tries a second code path for conv_dnn
Minor cleanups suggested by @nouiz
Nicer metaopt verbose output and conv_meta.verbose config flag to enable it
Turn off test_value when checking for beta init.
Merge pull request #2290 from abergeron/use_clang

Use clang++ on mac (if present).
Merge pull request #2266 from f0k/metaopt

conv2d meta-optimizer
Merge pull request #2293 from abergeron/fix_blas_tv

Turn off test_value when checking for beta init.