env.sh: Use a more portable way of counting online processors

getconf _NPROCESSORS_ONLN should be available on Linux, FreeBSD
and OSX at least:

http://linux.die.net/man/1/getconf
https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/getconf.1.html

getconf NPROCESSORS_ONLN is BSD variant which Haiku provides.

One might want to handle Solaris too some day...
cf. https://gist.github.com/jj1bdx/5746298

Docs/env.sh

@@ -22,7 +22,7 @@ if [ "x${TARGET_WORKSPACE}" = "x" ]; then
 fi
 
 if [ "x${USE_CPUS}" = "x" ]; then
-    NCPUS=$(grep -c "^processor" /proc/cpuinfo 2>/dev/null)
+    NCPUS=$(getconf _NPROCESSORS_ONLN 2>/dev/null || getconf NPROCESSORS_ONLN 2>/dev/null)
     NCPUS="${NCPUS:-1}"
     NCPUS=$((NCPUS * 2))
     USE_CPUS="-j${NCPUS}"