Fix pkgconfig file

The curerrent implementation does not work on systems, like e.g.
Debian, where the pkgconfig file is installed in
"/usr/lib/x86_64-linux-gnu/pkgconfig/blosc2.pc".

Since the current implementation assumes "prefix=${pcfiledir}/../.."
(corretponding to "/user/lib" in this case), the path to include files
("includedir=${prefix}/include") results to be not correct.

The fix has been only tested on Debian.

Author: avalentino

blosc2.pc.in

@@ -1,6 +1,5 @@
-prefix=${pcfiledir}/../.. # directory containing blosc2.pc
-libdir=${prefix}/lib
-includedir=${prefix}/include
+libdir=@CMAKE_INSTALL_PREFIX@/@CMAKE_INSTALL_LIBDIR@
+includedir=@CMAKE_INSTALL_PREFIX@/@CMAKE_INSTALL_INCLUDEDIR@
 
 Name: blosc2
 Description: A blocking, shuffling and lossless compression library