Forward all flags to try_compile() when cross-compiling

Poppler overrides the user-provided CMAKE_{C,CXX}_FLAGS and appends them to the per-configuration variables instead. This behaviour currently causes cross-compilation checks to fail since a CMake issue means these these per-configuration flags are no passed to try_compile() commands. In my case the flags specified in the toolchain file (as part of CMAKE_{C,CXX}_FLAGS_INIT) are absolutely required to compile successfully since a missing -mabi=/-march= flag will result in a linker error due to trying to link incompatible libraries. If CMAKE_TRY_COMPILE_CONFIGURATION is empty CMake will no propagate the per-configuration flags to try_compile() so we have to to set the value explicitly to one of the configurations that includes the user-provided CMAKE_C_FLAGS.

This is an upstream CMake issue that I've reported as https://gitlab.kitware.com/cmake/cmake/-/issues/22414 and https://gitlab.kitware.com/cmake/cmake/-/issues/19512.