The Laplacian pyramid can only work on images larger than 3x3 due to the
size of its convolution kernel. So if the image is too small return an
error (-1) before attempting to construction the pyramid.
The float version of many math functions were introduced in C99, and were
causing compile failure on systems like OS X. We now define them to their
double variant if __USE_ISOC99 is not defined. We may want to expand it later
to cover non-gcc compilers too, but since this is pdiff only, it's not really
important.
