Change libtool c:r:a from 3:0:0 to 4:0:1

Update libtool c:r:a for the libaom 3.1.0 release. The last public
release is libaom 3.0.0, which has current=3, revision=0, age=0.

The six rules in libtool.html#Updating-version-info are applied as
follows:
  Rule 1: N/A
  Rule 2: N/A
  Rule 3: Yes, so increment revision from 0 to 1.
  Rule 4: Yes, so increment current from 3 to 4, and set revision to 0.
  Rule 5: Yes, so increment age from 0 to 1.
  Rule 6: No.

The net change is: set revision to 0, bump current and age.

BUG=aomedia:3028

Change-Id: I7ab5a42a0a49b6f3a0dc4a57af2bbf669238e7af

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index e86ab95..18190f6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -42,9 +42,9 @@
 # passed to libtool.
 #
 # We set SO_FILE_VERSION = [c-a].a.r
-set(LT_CURRENT 3)
+set(LT_CURRENT 4)
 set(LT_REVISION 0)
-set(LT_AGE 0)
+set(LT_AGE 1)
 math(EXPR SO_VERSION "${LT_CURRENT} - ${LT_AGE}")
 set(SO_FILE_VERSION "${SO_VERSION}.${LT_AGE}.${LT_REVISION}")
 unset(LT_CURRENT)