Squashed 'third_party/eigen/' content from commit 61d72f6

Change-Id: Iccc90fa0b55ab44037f018046d2fcffd90d9d025
git-subtree-dir: third_party/eigen
git-subtree-split: 61d72f6383cfa842868c53e30e087b0258177257
diff --git a/eigen3.pc.in b/eigen3.pc.in
new file mode 100644
index 0000000..c5855de
--- /dev/null
+++ b/eigen3.pc.in
@@ -0,0 +1,6 @@
+Name: Eigen3
+Description: A C++ template library for linear algebra: vectors, matrices, and related algorithms
+Requires:
+Version: ${EIGEN_VERSION_NUMBER}
+Libs:
+Cflags: -I${INCLUDE_INSTALL_DIR}