Fix auto-detection of optional libraries for OS X

The OS X version of grep doesn't support -P (--perl-regexp). It was breaking the building.
Solved by slightly changing the RegExp and using -E (--extended-regexp).

util/has_lib.sh

@@ -1,15 +1,15 @@
 #!/usr/bin/env bash
 has_lib() {
-  local regex="lib$1.+(so|dylib)(?!\.)"
+  local regex="lib$1.+(so|dylib)$"
 
   # Try using ldconfig on linux systems
-  for LINE in `which ldconfig > /dev/null && ldconfig -p 2>/dev/null | grep -P $regex`; do
+  for LINE in `which ldconfig > /dev/null && ldconfig -p 2>/dev/null | grep -E $regex`; do
     return 0
   done
 
   # Try just checking common library locations
   for dir in /lib /usr/lib /usr/local/lib /opt/local/lib; do
-    test -d $dir && ls $dir | grep -P $regex && return 0
+    test -d $dir && ls $dir | grep -E $regex && return 0
   done
 
   return 1