cmake: mark internal cache vars as advanced

`CONFIG_OS_SUPPORT` and `CONFIG_SHARED` are marked this way in this
change. `CONFIG_SHARED` is also labeled as an internal flag to avoid
confusion with `BUILD_SHARED_LIBS`.

Bug: 437872594
Fixed: 437872594

Change-Id: I4617c746e1776f41c719d24374b2221407f8ffeb

cmake/aom_config_defaults.cmake

diff --git a/cmake/aom_config_defaults.cmake b/cmake/aom_config_defaults.cmake
index 6546b43..d029c73 100644
--- a/cmake/aom_config_defaults.cmake
+++ b/cmake/aom_config_defaults.cmake
@@ -96,6 +96,7 @@
                    "Build with temporal denoising support.")
 set_aom_config_var(CONFIG_MULTITHREAD 1 "Multithread support.")
 set_aom_config_var(CONFIG_OS_SUPPORT 0 "Internal flag.")
+mark_as_advanced(CONFIG_OS_SUPPORT)
 set_aom_config_var(CONFIG_PIC 0 "Build with PIC enabled.")
 set_aom_config_var(CONFIG_QUANT_MATRIX 1
                    "Build with quantization matrices for AV1 encoder."
@@ -103,7 +104,8 @@
 set_aom_config_var(CONFIG_REALTIME_ONLY 0
                    "Build for RTC-only. See aomcx.h for all disabled features.")
 set_aom_config_var(CONFIG_RUNTIME_CPU_DETECT 1 "Runtime CPU detection support.")
-set_aom_config_var(CONFIG_SHARED 0 "Build shared libs.")
+set_aom_config_var(CONFIG_SHARED 0 "Internal flag.")
+mark_as_advanced(CONFIG_SHARED)
 set_aom_config_var(CONFIG_WEBM_IO 1 "Enables WebM support.")
 
 # Debugging flags.