Fix build on systems where the libdir is lib64

[lgarrison]

Following the installation instructions, when I run make, the build stops with the error:

[ 60%] Building C object CMakeFiles/blosc_filter_shared.dir/src/blosc_filter.c.o
make[2]: *** No rule to make target 'blosc/lib/libblosc.so', needed by 'libblosc_filter.so'.  Stop.
make[1]: *** [CMakeFiles/Makefile2:115: CMakeFiles/blosc_filter_shared.dir/all] Error 2
make: *** [Makefile:146: all] Error 2

The cause seems to be that blosc is installing libraries to lib64, while hdf5-blosc looks for them in the hard-coded lib directory. Following https://github.com/Blosc/c-blosc/pull/328, I think the fix is just to use GnuInstallDirs to find the name of the libdir.

[James11222]

This worked!