Guard visibility variables on AIX

The addition of these visibility variables caused a test-suite build failure on AIX because visibility options aren't supported. This patch guards the variables on AIX.

Reviewed By: daltenty

Differential Revision: https://reviews.llvm.org/D122316

Author: jakeegan

MicroBenchmarks/libs/benchmark/CMakeLists.txt

@@ -51,8 +51,10 @@ option(BENCHMARK_USE_BUNDLED_GTEST "Use bundled GoogleTest. If disabled, the fin
 option(BENCHMARK_ENABLE_LIBPFM "Enable performance counters provided by libpfm" OFF)
 
 # Export only public symbols
-set(CMAKE_CXX_VISIBILITY_PRESET hidden)
-set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
+if(NOT ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
+  set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+  set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
+endif()
 
 if(MSVC)
     # As of CMake 3.18, CMAKE_SYSTEM_PROCESSOR is not set properly for MSVC and