Minor build improvement and fabs() bug fix (probably)

[jmarshall]

Hi Daniel,

A small build improvement, and a warning fix that changes wiggletools abs output for fractions — hopefully for the better:

• Use the standard CPPFLAGS and LDFLAGS as appropriate in the Makefile, so that users and packagers can easily tweak the build if necessary. (e.g. Bioconda will be able to use this to simplify their wiggletools recipe.)

• Fix abs/fabs warnings in unaryOps.c — this alters the output of wiggletools abs foo.wig for lines with fractional values (if that can happen in these files), outputting the value as input rather than truncating it to an integer — which is hopefully an improvement.

[dzerbino]

Hello @jmarshall ,

Hope all is well with you.

Thanks as always for your C advice,

Daniel