eigen: Fix pkg-config file prefix error

eigen generates	a pkg-config file which	has a broken prefix
(@CMAKE_INSTALL_PREFIX@).  This broken prefix causes an incorrect path when
other packages call pkg-config --cflags eigen.

This patch fixes the prefix in the generated eigen pc file, so projects
which depend on this pc file can now correctly find the eigen include
directory at build time.

Also correct the Cflags output to use the runtime prefix instead of the
build time STAGING_DIR, like we do elsewhere.

[Peter: drop backslashes, tweak commit message]
Signed-off-by: Matt Flax <flatmax@flatmax.org>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>