cmake: expose BUILD_SHARED_LIBS in cmake-gui

This change exposes `BUILD_SHARED_LIBS`, which controls whether shared
libraries are built. This should be used instead of `CONFIG_SHARED`,
which is an internal configuration variable that does not change the
build targets. More work is needed to mark certain variables, like this
one, as advanced (`mark_as_advanced()`) to avoid confusion.

Bug: 437872594
Change-Id: I027c00f2eaaff96205a4552ecc8c816f561c2dce

cmake/aom_config_defaults.cmake

diff --git a/cmake/aom_config_defaults.cmake b/cmake/aom_config_defaults.cmake
index 4590ed4..6546b43 100644
--- a/cmake/aom_config_defaults.cmake
+++ b/cmake/aom_config_defaults.cmake
@@ -14,6 +14,13 @@
 # config variables are added to the CMake variable cache via the macros provided
 # in util.cmake.
 
+# Only run if this is the main project.
+if(CMAKE_PROJECT_NAME STREQUAL "AOM")
+  # BUILD_SHARED_LIBS is a standard CMake variable, but we declare it here to
+  # make it prominent in the GUI.
+  option(BUILD_SHARED_LIBS "Build shared libraries (DLLs)." OFF)
+endif()
+
 #
 # The variables in this section of the file are detected at configuration time,
 # but can be overridden via the use of CONFIG_* and ENABLE_* values also defined