[third-party] Restrict DRM-dependent targets

This change restricts magma DRM backend to only build via CMake when
DOWNLOAD dependency resolution is selected. It also adds a warning
if the drm_headers target is undefined after dependency resolution.

Bug: b/279914930

Test: presubmit
Change-Id: I02dce28b2db66eb6a762a5ded60893aa806ee054
diff --git a/stream-servers/magma/CMakeLists.txt b/stream-servers/magma/CMakeLists.txt
index 1c1770d..caf55ca 100644
--- a/stream-servers/magma/CMakeLists.txt
+++ b/stream-servers/magma/CMakeLists.txt
@@ -1,4 +1,4 @@
-if (LINUX)
+if (LINUX AND DEPENDENCY_RESOLUTION STREQUAL "DOWNLOAD")
     set(
         gfxstream-magma-server-backend-sources
         IntelDrmDecoder.cpp)
diff --git a/third-party/CMakeLists.txt b/third-party/CMakeLists.txt
index 088f3bb..c99097d 100644
--- a/third-party/CMakeLists.txt
+++ b/third-party/CMakeLists.txt
@@ -317,3 +317,6 @@
 if(NOT TARGET flatbuffers)
     message(FATAL_ERROR "The dependency flatbuffers not found.")
 endif()
+if(NOT TARGET libdrm_headers)
+    message(FATAL_ERROR "The dependency libdrm_headers not found.")
+endif()