Issue locating gflags header file on Fedora

Trying to build OpenSfM on my local Fedora (32) system.

python setup.py build

Results the following error:

...
-- Found installed version of gflags: /usr/lib64/cmake/gflags
-- Detected gflags version: 2.2.2
CMake Error at cmake/FindGflags.cmake:118 (message):
  Failed to find gflags - Could not find file: //include/gflags/gflags.h
  containing namespace information in gflags install located at: //include.
Call Stack (most recent call first):
  cmake/FindGflags.cmake:215 (gflags_report_not_found)
  cmake/FindGflags.cmake:398 (gflags_check_gflags_namespace_using_regex)
  CMakeLists.txt:39 (find_package)
...

Like you can see gflags was detected, however the header file cannot be found.
The header file is located on the system in the following location: /usr/include/gflags/gflags.h

This issue can be solved by adding the following line under regex detect section:

function(GFLAGS_CHECK_GFLAGS_NAMESPACE_USING_REGEX)
...
  set(GFLAGS_INCLUDE_DIR /usr/include/)

But that of course will break other things and should properly fixed.

It's worth to mention that /usr/include is added to GFLAGS_CHECK_INCLUDE_DIRS but only when if (NOT GFLAGS_FOUND), however in my case, it was found.

Should be easy fix, but I didn't manage to figure this out yet.

Start with cmake/FindGflags.cmake, especially the regex detection function and the GFLAGS_CHECK_INCLUDE_DIRS handling described in the issue. Reproduce the failure with python setup.py build on Fedora and compare the detected gflags installation with /usr/include/gflags/gflags.h. Done means the build locates the header without a Fedora-specific hardcoded path.