Hide non-API functions when position independent code is enabled

Usage of -fvisibility=hidden is the recommended default for DSO's.

If the spdlog build is for a shared library, then API symbols have
SPDLOG_API marking on them, and cause the symbol to be exported, with
other symbols now hidden by default.

If the build is for a static library with position-independent code, then
spdlog is to be embedded as implementation detail in another shared
library or executable. In that case we don't want to export any symbol.
Conveniently, SPDLPG_API in that case resolves to nothing, so with the
default of hidden all symbols are made hidden.

CMakeLists.txt

@@ -141,6 +141,11 @@ if(SPDLOG_BUILD_PIC)
     set(CMAKE_POSITION_INDEPENDENT_CODE ON)
 endif()
 
+if((${CMAKE_POSITION_INDEPENDENT_CODE}) OR (${CMAKE_CXX_FLAGS} MATCHES "-fPIC"))
+    set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+    set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
+endif()
+
 find_package(Threads REQUIRED)
 message(STATUS "Build type: " ${CMAKE_BUILD_TYPE})
 # ---------------------------------------------------------------------------------------