Invalid CMake Syntax

[Daniel Mohan Sahu]

When attempting a cmake build with the default apt installed `libceres-dev` I encountered the following error:

```
CMake Error at /usr/lib/cmake/ceres/CeresConfig.cmake:89 (else):
  Flow control statements are not properly nested.
Call Stack (most recent call first):
  CMakeLists.txt:26 (find_package)
```

Digging into `/usr/lib/cmake/ceres/CeresConfig.cmake:89`, it looks like line 87 should be `elseif`, not `else`. Manually editing the file and rebuilding fixes my build.

```
85 if (Ceres_FIND_QUIETLY)
86 message(STATUS "Failed to find Ceres - " ${REASON_MSG} ${ARGN})
87 else (Ceres_FIND_REQUIRED)
88 message(FATAL_ERROR "Failed to find Ceres - " ${REASON_MSG} ${ARGN})
89 else()
```

I'm not sure how this got mangled as the source code version seems to be right (see link). We've also been using this package / installation method for quite a while without any issues, so I believe this is a new problem (as of 03/23).
https://github.com/ceres-solver/ceres-solver/blob/master/cmake/CeresConfig.cmake.in

Other information:
# lsb_release -rd
Description: Ubuntu 16.04.7 LTS
Release: 16.04

# apt-cache policy libceres-dev
libceres-dev:
  Installed: 1.12.0+dfsg0-5
  Candidate: 1.12.0+dfsg0-5
  Version table:
 *** 1.12.0+dfsg0-5 100
        100 /var/lib/dpkg/status