Use GNU installation directories

Use standard installation directories with GNUInstallDirs module which
also handle multilib paths.

Paths defined by GNUInstallDirs can also be customized passing arguments
to the CMake command line.
Fixes bugs in create_pkgconfig_file cmake macro

Bugs introduced by commit 5a3027c93eb5f234537d0d58cc87df2fa80f5f3a
${prefix} was missing in ${libdir} and ${includedir} variables.
${CMAKE_INSTALL_LIBDIR} and ${CMAKE_INSTALL_INCLUDEDIR} cannot be escaped
in order to be substituted by their values.

Signed-off-by: Luís Pereira <luis.artur.pereira@gmail.com>
Merge branch 'plfiorini-wip/001-gnudirs'

* plfiorini-wip/001-gnudirs:
  Fixes bugs in create_pkgconfig_file cmake macro
  Use GNU installation directories