gRPC requires abseil, but abseil can be statically linked to avoid installing
and tracking a ton of extra shared libs. Unfortunately, grpcpp headers now
include absl headers, so these need to be installed. Put them in a private
location because the full abseil installation is not provided.

diff -ur third_party/abseil-cpp/CMakeLists.txt third_party/abseil-cpp/CMakeLists.txt
--- a/third_party/abseil-cpp/CMakeLists.txt
+++ b/third_party/abseil-cpp/CMakeLists.txt
@@ -46,6 +46,7 @@
 OPTION(BUILD_TESTING "Build tests" OFF)
 
 project(absl LANGUAGES CXX VERSION 20210324)
+set(BUILD_SHARED_LIBS OFF)
 include(CTest)
 
 # Output directory is correct by default for most build setups. However, when
@@ -57,7 +58,7 @@
 # when absl is included as subproject (i.e. using add_subdirectory(abseil-cpp))
 # in the source tree of a project that uses it, install rules are disabled.
 if(NOT CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
-  option(ABSL_ENABLE_INSTALL "Enable install rule" OFF)
+  option(ABSL_ENABLE_INSTALL "Enable install rule" ON)
 else()
   option(ABSL_ENABLE_INSTALL "Enable install rule" ON)
 endif()
@@ -176,7 +177,7 @@
   endif()  # absl_VERSION
 
   install(DIRECTORY absl
-    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/grpc_absl
     FILES_MATCHING
       PATTERN "*.inc"
       PATTERN "*.h"
