Change version number to 2.3.1.
Fix builds of pigzt and pigzn to include zopfli, at -lm.

On some systems -lm is needed to link in the log function.
Clean up spacing, line lengths.
Respect LDFLAGS in Makefile.  Use CFLAGS consistently.
Avoid unused variable warning for NOTHREAD.
Add memory allocation tracking to pigz, including yarn and zlib.
Fix casting error in uncompressed length calculation.
Update zopfli to current google state.

git clone https://code.google.com/p/zopfli/
This includes the exposure of DeflatePart, though renames it to
ZopfliDeflatePart.  Also InitOptions is renamed ZopfliInitOptions,
as well as the data type to ZopfliOptions.  This version fixes
the determinism problem by initializing the random number
generator on each ZopfliDeflatePart call to the same values, and
furthermore makes the random state thread local, avoiding static
variables.
Update pigz to changed zopfli interface.

Also move ZopfliInitOptions to zopfli/deflate.c in order to avoid
linking in unnecessary modules.
Support zopfli in single thread case.
Add -F, -I, -M, and -O options for zopfli compression tuning.
Add -F, -I, -M, and -O options to documentation.
pigz version 2.3.1