[C++] Add support for system GFlags

[asfimport]

Reporter: Kouhei Sutou / @kou
Assignee: Kouhei Sutou / @kou

PRs and other links:

• GitHub Pull Request #3675

_{Note: This issue was originally created as ARROW-4599. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 3675
#3675

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
This has broken the build on linux with thirdparty gflags:

CMake Error at cmake_modules/ThirdpartyToolchain.cmake:658 (target_compile_definitions):
  Cannot specify compile definitions for imported target "gflags_static".
Call Stack (most recent call first):
  CMakeLists.txt:506 (include)

[asfimport]

Kouhei Sutou / @kou:
Could you open a new issue?
And please show your CMake version in the new issue.

[asfimport]

Francois Saint-Jacques / @fsaintjacques:
https://issues.apache.org/jira/browse/ARROW-4660