build: set CMAKE_INSTALL_LIBDIR to flux-core libdir

Problem: If flux-core was configured with a different libdir, then
Fluxion modules can't be found by default.

Ensure that CMAKE_INSTALL_LIBDIR matches the flux-core libdir by
using FLUX_CORE_LIBDIR when flux-core is found by pkgconfig.

Fixes #1092

CMakeLists.txt

@@ -58,6 +58,8 @@ find_package(FluxCore REQUIRED)
 if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
   message(STATUS "flux core found and no specific prefix specified, using ${FLUX_CORE_PREFIX}")
   set(CMAKE_INSTALL_PREFIX ${FLUX_CORE_PREFIX} CACHE PATH "..." FORCE)
+  message(STATUS "Using flux-core libdir of ${FLUX_CORE_LIBDIR}")
+  set(CMAKE_INSTALL_LIBDIR ${FLUX_CORE_LIBDIR} CACHE PATH "..." FORCE)
 endif()
 if(DEFINED ENV{PYTHON})
   set(Python_EXECUTABLE $ENV{PYTHON} CACHE FILEPATH "")