make new math code compatible with unused variable warning/error

make new math code compatible with unused variable warning/error

commit b50d315fd23f0fbc4c11e2583801dd123d933745 introduced
fp_force_eval implemented by default with a dead store to a volatile
variable. unfortunately introduces warnings with -Wunused-variable and
breaks the ability to use -Werror with the default warning options set
by configure when warnings are enabled.

we could just call fp_barrier instead, but that results in a spurious
load after the store due to volatile semantics.

the fix committed here avoids the load. it will still produce warnings
without -Wno-unused-but-set-variable, but that's part of our default
warning profile, and there are already other locations in the source
where an unused variable warning will occur without it.