Enable shared libraries with NVHPC >= 25.7

[pmarguinaud]

Shared libraries are working with nvhpc 25.7; this PR will enable them.

[awnawab]

Thanks a lot @pmarguinaud for testing and contributing this, it's great to be able to have shared libs with offload enabled. This is ok in principle, but it removes the ability for the user to manually ask for static builds. I propose something like this instead:

set( SHARED_LIBS_FOR_OFFLOAD_DEFAULT OFF )
if( CMAKE_Fortran_COMPILER_ID MATCHES "PGI|NVHPC" AND
    CMAKE_Fortran_COMPILER_VERSION VERSION_GREATER_EQUAL 25.7.0)
    set( SHARED_LIBS_FOR_OFFLOAD_DEFAULT ON )
endif()
cmake_dependent_option(BUILD_SHARED_LIBS "Dynamically link field_api" ON "NOT (HAVE_ACC OR HAVE_OMP_OFFLOAD)" ${SHARED_LIBS_FOR_OFFLOAD_DEFAULT})

if( (HAVE_ACC OR HAVE_OMP_OFFLOAD) AND NOT ${SHARED_LIBS_FOR_OFFLOAD_DEFAULT} )
  ecbuild_warn("Enabling GPU offload forces static linking.")
endif()