[petsc-dev] libm autodetection broken with -ffast-math

[Jed Brown]

"gcc -O3 -ffast-math" and "clang -O3 -fast-math" remove the calls in our
math checks, resulting in configure thinking that -lm is not required.
To fix the test, we have to ensure that the functions get called with
values not known at compile time, which will make the configure code
more complicated/less generic.  This may become an issue in other places
if compilers learn more about side-effect-free functions appearing in
libraries.

In the mean-time, you can configure with LIBS=-lm.
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