Gentoo currently uses unversioned lua:0 - lua.pc instad of lua5.1.pc, /usr/bin/lua instead of /usr/bin/lua5.1

diff --git a/configure.ac b/configure.ac
index 810a288..51ca607 100644
--- a/configure.ac
+++ b/configure.ac
@@ -305,9 +305,9 @@ else
     fi
 
     if test "x$found_lua51" != "xyes"; then
-        PKG_CHECK_EXISTS([lua5.1 >= $LUA51_REQUIRED], [
+        PKG_CHECK_EXISTS([lua >= $LUA51_REQUIRED], [
             found_lua51=yes
-            with_lua51=lua5.1
+            with_lua51=lua
         ], [
             found_lua51=no
         ])