Fix Makefile CUDA_VERSION extraction on OSX Yosemite

[yalesong]

The current way of extracting CUDA_VERSION causes problem on OSX Yosemite

$ make
awk: syntax error at source line 1
 context is
    {exit  >>>  < <<<  7.0;}
awk: illegal statement at source line 1

$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.10.5
BuildVersion:   14F27

$ uname -prs
Darwin 14.5.0 i386

Looks like the change suggested in #4075 wasn't helpful in my case: grep after the second pipe doesn't work as expected:

$ echo "nvcc: ... release 7.5, V7.5.19" | grep -o 'release [0-9.]*'
release 7.5
$ echo "nvcc: ... release 7.5, V7.5.19" | grep -o 'release [0-9.]*' | grep -o '[0-9.]*'

The following did work for me:

$ echo "nvcc: ... release 7.5, V7.5.19" | grep -o 'release [0-9.]*' | grep -o '[0-9].*'
7.5
$ echo "nvcc: ... release 7.5, V7.5.19" | grep -o 'release [0-9.]*' | tr -d '[a-z ]'
7.5
$ echo "nvcc: ... release 7.5, V7.5.19" | grep -o 'release [0-9.]*' | sed 's/[a-z ]//g'
7.5

[longjon]

Ah, the perils of regular expressions. I think what's happening here is that [0-9.]* matches the empty string at the beginning instead of the desired digits (note that grep -c does report a match). I don't know why the behavior is different with GNU grep.

I'll accept this patch; it would be a little nicer if the latter part of the pipeline did not depend on the former, but this works.

Thanks!