Incorrect flag name in CeresConfig.cmake

[Tanner Ray]

1. Description: Ubuntu 20.04.5 LTS
   Release: 20.04
2. libceres-dev 1.14.0-4ubuntu1.1
3. Expected cmake project to build and find_package() to work
4. find_package() failed

 The error that is thrown is below:

Found installed version of Eigen: /usr/lib/cmake/eigen3
-- Found required Ceres dependency: Eigen version 3.3.7 in /usr/include/eigen3
-- Found required Ceres dependency: glog
CMake Error at /usr/lib/cmake/Ceres/CeresConfig.cmake:88 (message):
  Failed to find Ceres - Missing required Ceres dependency: gflags. Searched
  using GFLAGS_INCLUDE_DIR_HINTS: and gflags_DIR:
  /usr/lib/x86_64-linux-gnu/cmake/gflags.
Call Stack (most recent call first):
  /usr/lib/cmake/Ceres/CeresConfig.cmake:312 (ceres_report_not_found)
  CMakeLists.txt:46 (find_package)

To fix this in /usr/lib/cmake/Ceres/CeresConfig.cmake replace GFLAGS_FOUND with Gflags_FOUND.

[Tanner Ray (tnr00071)]

I added an issue to the ceres-solver github as well.

https://github.com/ceres-solver/ceres-solver/issues/946