On Linux* and Mac OS* systems, libimf.a is the math library provided by Intel. This is in addition to libm.a, which is the math library provided with gcc*.
Both of these libraries are linked in by default because certain math functions supported by the GNU math library are not available in the Intel math library. This linking arrangement allows the GNU users to have all functions available when using ifort, with Intel optimized versions available when supported.
libimf.a is linked in before libm.a. If you link in libm.a first, it will change the versions of the math functions that are used.
On Windows* systems, Intel provides libm.lib (static library) and libmmd.dll (the DLL version).