Added the optional Milo Yip DTOA library (emyg_dtoa) to avoid
issues where the standard sprintf() dtoa function changes output
based on locale settings. It is also 40-50% faster than the
standard dtoa for raw numeric data.
If you wish to use this third party library you can compile
libxlsxwriter with it by passing `USE_DTOA_LIBRARY=1` to
make. The USE_DOUBLE_FUNCTION build variable is no longer used.
Imported source from https://github.com/miloyip/dtoa-benchmark
Feature request #272
