[abseil] Fix CMake warning (#12781)

* abseil: Patch to fix CMake Threads dependency warning.

* abseil: Make portfile comment more concise

ports/abseil/fix-cmake-threads-dependency.patch

@@ -0,0 +1,13 @@
+diff --git a/CMake/abslConfig.cmake.in b/CMake/abslConfig.cmake.in
+index 60847fa77..6d23f63d3 100644
+--- a/CMake/abslConfig.cmake.in
++++ b/CMake/abslConfig.cmake.in
+@@ -1,6 +1,7 @@
+ # absl CMake configuration file.
+
+-include(FindThreads)
++include(CMakeFindDependencyMacro)
++find_dependency(Threads)
+
+ @PACKAGE_INIT@
+

ports/abseil/portfile.cmake

@@ -7,9 +7,12 @@ set(ABSEIL_PATCHES
 
     # This patch is an upstream commit, the related PR: https://github.com/abseil/abseil-cpp/pull/637
     fix-MSVCbuildfail.patch
-    
+
     # Remove this patch in next update, see https://github.com/google/cctz/pull/145
     fix-arm-build.patch
+
+    # This patch is an upstream commit: https://github.com/abseil/abseil-cpp/commit/68494aae959dfbbf781cdf03a988d2f5fc7e4802
+    fix-cmake-threads-dependency.patch
 )
 
 if("cxx17" IN_LIST FEATURES)

ports/abseil/vcpkg.json

@@ -1,7 +1,7 @@
 {
   "name": "abseil",
   "version-string": "2020-03-03",
-  "port-version": 7,
+  "port-version": 8,
   "description": [
     "an open-source collection designed to augment the C++ standard library.",
     "Abseil is an open-source collection of C++ library code designed to augment the C++ standard library. The Abseil library code is collected from Google's own C++ code base, has been extensively tested and used in production, and is the same code we depend on in our daily coding lives.",