CMakeLists.txt overrides CMAKE_C_COMPILER forcing clang-18 when IVAS is included as a subproject
The lines:
```cmake
# set C compiler
find_program(cc NAMES clang-18 clang REQUIRED)
set(CMAKE_C_COMPILER "${cc}" CACHE STRING "")
```
override any existing configuration, forcing parent projects to also use `clang-18`. This causes issues when IVAS is used in a parent project via `add_subdirectory()`.
The solution is to only set this if IVAS is being built standalone:
```cmake
if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR AND NOT CMAKE_C_COMPILER)
find_program(cc NAMES clang-18 clang REQUIRED)
set(CMAKE_C_COMPILER "${cc}" CACHE FILEPATH "C compiler")
endif()
```
issue