Use find_package(absl CONFIG)

Same approach as in Protobuf:
bc127f2b56/cmake/abseil-cpp.cmake (L36)

cpp/CMakeLists.txt

@@ -106,7 +106,8 @@ if (USE_ALTERNATE_FORMATS)
 endif ()
 
 # Find all the required libraries and programs.
-find_package(absl)
+# Use "CONFIG" as there is no built-in cmake module for absl.
+find_package(absl CONFIG REQUIRED)
 
 if(NOT absl_FOUND)
   # Overide abseil install rules for subprojects

cpp/cmake/config.cmake.in

@@ -2,7 +2,7 @@
 
 include(CMakeFindDependencyMacro)
 
-find_dependency(absl)
+find_dependency(absl CONFIG)
 find_dependency(Protobuf)
 
 include("${CMAKE_CURRENT_LIST_DIR}/@targets_export_name@.cmake")