Strip the wheel-deployment RPATH from the Linux build.

MakeLib.cmake sets `-rpath,$ORIGIN` on every BornAgain library so the
manylinux wheel layout can dlopen vendored deps from `bornagain/lib/`.
For distro packaging the libs land in /usr/lib64/ alongside system
Qt/cerf/etc., so the $ORIGIN rpath 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.

(24.0 dropped the `$ORIGIN/../extra` component the 23.0 block carried,
but the Linux rpath is still set and still wrong for distro packaging.)

verified 2026-05-28 against bornagain-24.0.

--- 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")
-        set_target_properties(${lib} PROPERTIES LINK_FLAGS ${link_flags})
-    elseif(APPLE)
+    # NOTE: the original LINUX block set RPATH=$ORIGIN so the wheel layout
+    # could dlopen vendored .so files from .../bornagain/lib. Distro packaging
+    # installs to /usr/lib64/ with system libs, so the bundled 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()