Revert "Fix(cmake): CMake doesn't find system installed SPIRV-Headers (#5422)" (#5517)

This reverts commit 7d2429594d84f09d0aef2c0a36d1a7234941a040.

external/CMakeLists.txt

diff --git a/external/CMakeLists.txt b/external/CMakeLists.txt
index 153676e..5d8a3da 100644
--- a/external/CMakeLists.txt
+++ b/external/CMakeLists.txt
@@ -27,26 +27,26 @@
 endfunction()
 
 if (DEFINED SPIRV-Headers_SOURCE_DIR)
-  add_subdirectory(${SPIRV-Headers_SOURCE_DIR})
-  set(SPIRV-Headers_INCLUDE_DIRS ${SPIRV-Headers_SOURCE_DIR}/include)
-elseif (IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/spirv-headers/)
-  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/spirv-headers/)
-  set(SPIRV-Headers_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/spirv-headers/include)
-elseif(IS_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}/SPIRV-Headers/)
-  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/SPIRV-Headers/)
-  set(SPIRV-Headers_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR}/SPIRV-Headers/include)
-elseif()
+  # This allows flexible position of the SPIRV-Headers repo.
+  set(SPIRV_HEADER_DIR ${SPIRV-Headers_SOURCE_DIR})
 else()
-  find_package(SPIRV-Headers QUIET)
-  if (SPIRV-Headers_FOUND)
-    get_target_property(SPIRV-Headers_INCLUDE_DIRS SPIRV-Headers::SPIRV-Headers INTERFACE_INCLUDE_DIRECTORIES)
-  else ()
-    message(FATAL_ERROR
-      "SPIRV-Headers was not found - please checkout a copy under external/.")
-  endif()
+  set(SPIRV_HEADER_DIR ${CMAKE_CURRENT_SOURCE_DIR}/spirv-headers)
 endif()
-set(SPIRV_HEADER_INCLUDE_DIR ${SPIRV-Headers_INCLUDE_DIRS} PARENT_SCOPE)
-message(STATUS "Found SPIRV-Headers: ${SPIRV-Headers_INCLUDE_DIRS}")
+
+if (IS_DIRECTORY ${SPIRV_HEADER_DIR})
+  # TODO(dneto): We should not be modifying the parent scope.
+  set(SPIRV_HEADER_INCLUDE_DIR ${SPIRV_HEADER_DIR}/include PARENT_SCOPE)
+
+  # Add SPIRV-Headers as a sub-project if it isn't already defined.
+  # Do this so enclosing projects can use SPIRV-Headers_SOURCE_DIR to find
+  # headers to include.
+  if (NOT DEFINED SPIRV-Headers_SOURCE_DIR)
+    add_subdirectory(${SPIRV_HEADER_DIR})
+  endif()
+else()
+  message(FATAL_ERROR
+    "SPIRV-Headers was not found - please checkout a copy under external/.")
+endif()
 
 if (NOT ${SPIRV_SKIP_TESTS})
   # Find gmock if we can. If it's not already configured, then try finding