magrathea: Tell cmELF about DT_RUNPATH (#13497)

Our Linux release machine 'magrathea' has a very old Linux to help build
portable binaries.  It is so old that <elf.h> does not define the
DT_RUNPATH constant.  Define the correct value in the build flags to
activate handling of ELF RUNPATH entries in the binary release.

Utilities/Release/magrathea_release.cmake

@@ -3,6 +3,8 @@ set(HOST magrathea)
 set(MAKE_PROGRAM "make")
 set(CC gcc332s)
 set(CXX c++332s)
+set(CFLAGS   -DDT_RUNPATH=29)
+set(CXXFLAGS -DDT_RUNPATH=29)
 set(INITIAL_CACHE "
 CMAKE_BUILD_TYPE:STRING=Release
 CURSES_LIBRARY:FILEPATH=/usr/i686-gcc-332s/lib/libncurses.a