On Linux, use --enable-new-dtags when specifying -rpath to linker.

This should allow LD_LIBRARY_PATH to work as desired.  Per trouble
report from Andy Colson.

src/makefiles/Makefile.linux

 AROPT = crs
 export_dynamic = -Wl,-E
-rpath = -Wl,-rpath,'$(rpathdir)'
+# Use --enable-new-dtags to generate DT_RUNPATH instead of DT_RPATH.
+# This allows LD_LIBRARY_PATH to still work when needed.
+rpath = -Wl,-rpath,'$(rpathdir)',--enable-new-dtags
 allow_nonpic_in_shlib = yes
 DLSUFFIX = .so