[C++] gflags EP build gets triggered (as a no-op) on subsequent calls to make or ninja build

[asfimport]

Since switching to Ninja for builds I've noticed this rough edge. It doesn't seem to affect other EPs. @xhochy have you also seen this?

$ ninja
[4/45] Performing configure step for 'gflags_ep'
-- Configuring done
-- Generating done
-- Build files have been written to: /home/wesm/code/arrow/cpp/test-build/gflags_ep-prefix/src/gflags_ep
[5/45] Performing build step for 'gflags_ep'
ninja: no work to do.
[6/45] Performing install step for 'gflags_ep'
[0/1] Install the project...
-- Install configuration: "DEBUG"
-- Up-to-date: /home/wesm/code/arrow/cpp/test-build/gflags_ep-prefix/src/gflags_ep/lib/cmake/gflags/gflags-config.cmake
-- Up-to-date: /home/wesm/code/arrow/cpp/test-build/gflags_ep-prefix/src/gflags_ep/lib/cmake/gflags/gflags-config-version.cmake
-- Up-to-date: /home/wesm/code/arrow/cpp/test-build/gflags_ep-prefix/src/gflags_ep/lib/cmake/gflags/gflags-targets.cmake
-- Installing: /home/wesm/code/arrow/cpp/test-build/gflags_ep-prefix/src/gflags_ep/lib/cmake/gflags/gflags-targets-debug.cmake
-- Up-to-date: /home/wesm/code/arrow/cpp/test-build/gflags_ep-prefix/src/gflags_ep/bin/gflags_completions.sh
[8/45] Building CXX object CMakeFiles/arrow_objlib.dir/src/arrow/ipc/json-internal.cc.o
<SNIP>

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

_{Note: This issue was originally created as ARROW-1113. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Yes, I'm also seeing this. The only special thing of gflags is that it isn't downloaded from a tarball but rather directly from git.

[asfimport]

Wes McKinney / @wesm:
That's probably it, then. Let's see if using a tarball fixes it

[asfimport]

Wes McKinney / @wesm:
PR: #776

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 776
#776