[libhipcxx] Fix -x hip scope: use target_compile_options not CMAKE_CXX_FLAGS

CMAKE_CXX_FLAGS=-x hip gets passed to the linker step too, causing
amdclang++ to try to parse ELF object files as HIP source:
  error: expected unqualified-id (reading ELF magic bytes)

Move -x hip to target_compile_options on the version_check target so
it only applies during compilation, not linking. Remove it from the
global cmake_opts passed to the sub-project configure step.

Co-Authored-By: Claude Sonnet 4 <noreply@anthropic.com>

Author: david-salinas

cmake/test/CMakeLists.txt

@@ -25,7 +25,6 @@ set(cmake_opts
   -D "CMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}"
   -D "CMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}"
   -D "CMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}"
-  -D "CMAKE_CXX_FLAGS=-x hip"
 )
 
 # Temporary installation prefix for tests against installed project:

cmake/test/test_export/CMakeLists.txt

@@ -23,6 +23,7 @@ find_package(hip REQUIRED)
 find_package(libhipcxx REQUIRED)
 
 add_executable(version_check version_check.cxx)
+target_compile_options(version_check PRIVATE -x hip)
 target_link_libraries(version_check PRIVATE libhipcxx::libhipcxx hip::host)
 enable_testing()
 add_test(NAME version_check COMMAND "$<TARGET_FILE:version_check>")