configure: use AC_SEARCH_LIBS instead of AC_CHECK_LIB

This avoids unneccessary -lm flags on some platforms integrating libm
into libc and allows us to detect systems that do not provide
implementations of math functions in either libc or libm.

The caveat is, that some systems, like FreeBSD split their math
functions between libc and libm. By checking for C99 lrint instead of
C89 fabs, this check correctly includes libm in FreeBSD; let's hope this
also holds for other systems splitting up their math.h implementations.

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