Hide symbols by default also with a system Abseil

The default hidden symbol visibility was only applied when building
against the bundled Abseil, so shared libraries built against a system
Abseil exported all internal symbols.

Set the visibility presets regardless of how Abseil is provided.

Change-Id: I23e1bb1e7eea0d5822806b227ccaa6089587786c

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6911897..05fea16 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -249,12 +249,6 @@
   unset(CMAKE_CXX_STANDARD)
   unset(CMAKE_CXX_VISIBILITY_PRESET)
 
-  # Set the default symbol visibility to hidden to unify the behavior among
-  # the various compilers and to get smaller binaries
-  set(CMAKE_C_VISIBILITY_PRESET hidden)
-  set(CMAKE_CXX_VISIBILITY_PRESET hidden)
-  set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
-
   set(CERES_USE_SYSTEM_ABSL NO)
 else()
   if (NOT absl_VERSION)
@@ -267,6 +261,12 @@
   endif()
   set(CERES_USE_SYSTEM_ABSL YES)
 endif()
+
+# Set the default symbol visibility to hidden to unify the behavior among
+# the various compilers and to get smaller binaries
+set(CMAKE_C_VISIBILITY_PRESET hidden)
+set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
 if (NOT absl_VERSION)
   install (CODE [=[message (FATAL_ERROR "Cannot generate CeresConfig.cmake due to missing Abseil version. Using resulting package configuration will likely result in integration errors. Set absl_VERSION manually to disable this error message.")]=])
 endif()