Tenacity generates RPATH values weirdly, and it doesn't play nice with portage:

1. CMAKE_RUNTIME_OUTPUT_DIRECTORY is used instead of CMAKE_INSTALL_PREFIX, so by
default the RPATH will be a directory under "/var/tmp/portage/..."

2. the INSTALL_RPATH will retain values from the BUILD_RPATH, leading to the
final binary always having the build directory appended in it's RPATH (even if
the above issue is fixed)

this patch both corrects the INSTALL_RPATH to use CMAKE_INSTALL_PREFIX
(INSTALL_PREFIX is an alias for it), and sets BUILD_WITH_INSTALL_RPATH
to prevent the BUILD_RPATH for being linked into the final binaries.

it is necessary to set BUILD_RPATH, even though it's not technically being
used, as there is a strange behaviour that overrides BUILD_WITH_INSTALL_RPATH
if this is unset or set to "".

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 345b4ca47..883a097bf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -363,9 +363,9 @@ set( _MODDIR "${_DEST}/modules" )
 set( _EXEDIR "${_DEST}" )
 
 # Setup RPATH handling
-set( CMAKE_BUILD_RPATH "${_DEST}/${_PKGLIB}" )
-set( CMAKE_BUILD_WITH_INSTALL_RPATH FALSE )
-set( CMAKE_INSTALL_RPATH "$ORIGIN/../${_PKGLIB}" )
+set( CMAKE_BUILD_RPATH "${INSTALL_PREFIX}/${_PKGLIB}" )
+set( CMAKE_BUILD_WITH_INSTALL_RPATH TRUE )
+set( CMAKE_INSTALL_RPATH "${INSTALL_PREFIX}/${_PKGLIB}" )
 set( CMAKE_INSTALL_RPATH_USE_LINK_PATH FALSE )
 
 # Adjust them for the Mac