Support Alpha architecture for processor_count

Alpha machines with Linux do have /proc/cpuinfo but its format is
different from normal Linux machines, and most specifically the
processor entry is missing. Moreover, Alpha machines can be configured
to compartimentalize their CPUs into different machines leading to
further confusion when reading /proc/cpuinfo.

Using /usr/bin/nproc seems to be the most reliable method of determining
the number of processors on Alpha. Since reading /proc/cpuinfo is not
reliable this patch places the nproc method before trying /proc/cpuinfo.

For futher reference see our downstream bug report at
https://bugs.gentoo.org/show_bug.cgi?id=587986

Author: graaff

CHANGELOG.md

@@ -1,3 +1,5 @@
+* Support Alpha with `Concurrent::processor_count`
+
 ## Current Release v1.0.2 (2 May 2016)
 
 * Fix bug with `Concurrent::Map` MRI backend `#inspect` method

lib/concurrent/utility/processor_counter.rb

@@ -28,6 +28,7 @@ def initialize
       # processor", which taked into account hyperthreading.
       #
       # * AIX: /usr/sbin/pmcycles (AIX 5+), /usr/sbin/lsdev
+      # * Alpha: /usr/bin/nproc (/proc/cpuinfo exists but cannot be used)
       # * BSD: /sbin/sysctl
       # * Cygwin: /proc/cpuinfo
       # * Darwin: /usr/bin/hwprefs, /usr/sbin/sysctl
@@ -84,6 +85,8 @@ def compute_processor_count
             result = WIN32OLE.connect("winmgmts://").ExecQuery(
               "select NumberOfLogicalProcessors from Win32_Processor")
             result.to_enum.collect(&:NumberOfLogicalProcessors).reduce(:+)
+          elsif File.executable?("/usr/bin/nproc")
+            IO.popen("/usr/bin/nproc --all").read.to_i
           elsif File.readable?("/proc/cpuinfo")
             IO.read("/proc/cpuinfo").scan(/^processor/).size
           elsif File.executable?("/usr/bin/hwprefs")