Simplify AC_LBL_CHECK_COMPILER_OPT a bit.

Rather than use -Werror=unknown-warning-option for clang, just use
-Werror. It allows to check options which are not warning options.
Use -ffloat-store while compiling if possible.

It makes life better w.r.t. GitHub bug #333. (Although the problem may persist
for unusual architectures.)
Merge pull request #334 from tejblum/float-store-2

As far as I can tell, these changes are correct and they fix GH #333.