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Some compilers may add their own RPATH entries when invoking the linker. For example, a GCC installation may contain the following definition in the specs file: *link_libgcc: %D -rpath <<some specific rpath in which libstdc++.so can be found>> In this case binaries may contain RPATH entries that CMake did not add. When we update the RPATH on installation we must preserve these entries even if CMake thinks the INSTALL_RPATH value should be empty. Fix this by always using file(RPATH_CHANGE) and teach it to behave as file(RPATH_REMOVE) if the actual RPATH in the file is empty after replacing the build-tree RPATH with the install-tree RPATH. This will preserve any compiler-added RPATH value instead of removing it.
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