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The build/real prefix handling using sed breaks if build != real and the
standard include / lib directories are used ($prefix/include and $prefix/lib).
E.G.
prefix_build="/usr", libdir="$prefix/lib", includedir="$prefix/include".
If this gets installed with make DESTDIR="/foo" install, then we end up with prefix_real = prefix = "/foo/usr" as expected, but
includedir="/foo/foo/usr/include" and libdir="/foo/foo/usr/lib" because of the double sed invocation (prefix is already expanded). Work around it by ensuring we only match the beginning of the string.