Add -fvisibility=hidden to GCC/Clang compiler flags

cmake/compilers/gnu.cmake

@@ -24,6 +24,10 @@ add_compile_options(-Wno-format-zero-length -Wno-format-nonliteral)
 # that rely on the compiler not optimising them away, so disable it
 add_compile_options(-fno-strict-aliasing)
 
+# This is necessary to hide all symbols unless explicitly exported
+# via the Q_EXPORT macro
+add_compile_options(-fvisibility=hidden)
+
 if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
     set(CMAKE_INSTALL_PREFIX /opt/quake3)
 endif()