Strip the wheel-deployment RPATH from the Linux build.

MakeLib.cmake sets `-rpath,$ORIGIN:$ORIGIN/../extra` on every BornAgain
library so the manylinux wheel layout can dlopen vendored deps from
`bornagain/lib/extra/`. For distro packaging the libs land in /usr/lib64/
alongside system Qt/cerf/etc., so `$ORIGIN/../extra` resolves to nothing
and Portage's RUNPATH QA check flags it. Remove the Linux RPATH; the macOS
@loader_path setup is left intact for upstream-style builds.

verified 2026-05-14 against bornagain-23.0.

--- a/cmake/BornAgain/MakeLib.cmake
+++ b/cmake/BornAgain/MakeLib.cmake
@@ -68,10 +68,11 @@

     # Set runtime-location of library dependencies
     # See our deployment paper (Nejati et al 2024) for explanation.
-    if(LINUX)
-        set(link_flags "-Wl,--disable-new-dtags,-rpath,\$ORIGIN:\$ORIGIN/../extra")
-        set_target_properties(${lib} PROPERTIES LINK_FLAGS ${link_flags})
-    elseif(APPLE)
+    # NOTE: the original Linux block sets RPATH=$ORIGIN:$ORIGIN/../extra so
+    # the wheel layout can dlopen vendored .so files from .../bornagain/lib/extra.
+    # Distro packaging installs to /usr/lib64/ with system libs, so the
+    # bundled-extra rpath is removed (it would trip RUNPATH QA checks).
+    if(APPLE)
         target_link_options(${lib} PRIVATE
             "-Wl,-rpath,@loader_path,-rpath,@loader_path/extra")
     endif()