Link shared library against libm and libpthread

kfunc.c uses libm and bgzf.c requires pthreads -- hat tip @apmasell.
Partial fix for #94.

(No changes needed for libhts.dylib, as on Mac OS X maths and pthreads
functions are in libSystem.dylib -- -lm and -lpthread merely add further
symlinks to libSystem.dylib to the linking.)
Create a pkg-config description for easy linking

Htslib-using programs can optionally take advantage of pkg-config, which
allows easy detection of the presence and version of libraries using the
PKG_CHECK_MODULES macro in GNU Autoconf.  It also allows easy compilation,
without concern for installation paths, using pkg-config as follows:

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

[Various modifications to Andre's original patch, notably to delay
expanding installation directory variables until install time, adding
an -uninstalled pkg-config file, and naming the @placeholders@ as per
their autoconf equivalents.  Closes #94.]