configure: Fix check for INSERT in linker script

GCC/Clang use local access when referencing a const variable,
so the conftest.so may have no dynamic relocation.
LLD reports `error: unable to insert .foo after .rela.dyn` when the
destination section does not exist.

Use a non-const int to ensure that .rela.dyn exists.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>