#96 Fix build on riscv64/FreeBSD

[Robert Clausecker]

Lame is currently BROKEN on riscv64 FreeBSD due to an unsupported
fpsetenv() call. RISC-V does not have floating point exceptions and nobody
bothered to stub out the function in the libc.

This is a patch by the LLVM project to unbreak the build on
platforms such as riscv64 with incomplete fpsetmask support.
On riscv64 specifically, there are no traps for invalid
floating point operations, so whatever this masking code
tries to achieve is meaningless anyway.

[Alexander Leidinger]

Committed. Thanks!