Remove forced CONFIG from find_package(Eigen3)

- Ceres will fail to configure if Eigen3::Eigen target is not found, and
  the minimum required Eigen version specified (3.3) exports Eigen as
  a CMake package and this is reflected in the default Ubuntu 18.04
  packages.
- This permits users to specify their own Eigen3 detection should they
  choose to do so, but they must do so via an imported target.

Change-Id: I5edff117c8001770004f49012ac1ae63b66ec9c1

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7354c43..33e98b1 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -239,8 +239,7 @@
 
 # Eigen.
 # Eigen delivers Eigen3Config.cmake since v3.3.3
-find_package(Eigen3 3.3 CONFIG REQUIRED
-  HINTS ${HOMEBREW_INCLUDE_DIR_HINTS})
+find_package(Eigen3 3.3 REQUIRED HINTS ${HOMEBREW_INCLUDE_DIR_HINTS})
 if (EIGEN3_FOUND)
   message("-- Found Eigen version ${EIGEN3_VERSION_STRING}: ${EIGEN3_INCLUDE_DIRS}")
   if (CMAKE_SYSTEM_PROCESSOR MATCHES "^(aarch64.*|AARCH64.*)" AND

cmake/CeresConfig.cmake.in

diff --git a/cmake/CeresConfig.cmake.in b/cmake/CeresConfig.cmake.in
index ae94833..e5e2976 100644
--- a/cmake/CeresConfig.cmake.in
+++ b/cmake/CeresConfig.cmake.in
@@ -191,7 +191,7 @@
 # Search quietly to control the timing of the error message if not found. The
 # search should be for an exact match, but for usability reasons do a soft
 # match and reject with an explanation below.
-find_package(Eigen3 ${CERES_EIGEN_VERSION} CONFIG QUIET)
+find_package(Eigen3 ${CERES_EIGEN_VERSION} QUIET)
 if (EIGEN3_FOUND)
   if (NOT EIGEN3_VERSION_STRING VERSION_EQUAL CERES_EIGEN_VERSION)
     # CMake's VERSION check in FIND_PACKAGE() will accept any version >= the

docs/source/installation.rst

diff --git a/docs/source/installation.rst b/docs/source/installation.rst
index db12cce..d727c31 100644
--- a/docs/source/installation.rst
+++ b/docs/source/installation.rst
@@ -444,7 +444,6 @@
 #. Try running ``Configure``. It won't work. It'll show a bunch of options.
    You'll need to set:
 
-   #. ``EIGEN_INCLUDE_DIR_HINTS``
    #. ``GLOG_INCLUDE_DIR_HINTS``
    #. ``GLOG_LIBRARY_DIR_HINTS``
    #. (Optional) ``SUITESPARSE_INCLUDE_DIR_HINTS``