Install eigen3.pc in default directory if pkgconfig not found (bug 358).
Install eigen3.pc in default directory if pkgconfig not found (bug 358).

ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,toastedcrumpets/eigen,Zefz/eigen,Zefz/eigen,ritsu1228/eigen,toastedcrumpets/eigen,TSC21/Eigen,pasuka/eigen,TSC21/Eigen,ritsu1228/eigen,pasuka/eigen,Zefz/eigen,ROCmSoftwarePlatform/hipeigen,ROCmSoftwarePlatform/hipeigen,TSC21/Eigen,Zefz/eigen,toastedcrumpets/eigen,toastedcrumpets/eigen,pasuka/eigen,TSC21/Eigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,ritsu1228/eigen,ritsu1228/eigen,pasuka/eigen