Link shared library against libm and libpthread

kfunc.c uses libm and bgzf.c rquires libpthread.
Create a pkg-config description for easy linking

pkg-config allows easy detection of the prescence and version of
libraries using the PKG_CHECK_MODULES macro in GNU AutoConf. It also
allows easy compilation, without concern for installation paths, using
the pkg-config command-line utility, as follows:

    cc -o tabix tabix.c `pkg-config --cflags --libs libhts`