[Buildroot] [PATCH v2] package/flann: fix build with cmake >= 3.11

[Peter Korsgaard]

>>>>> "Romain" == Romain Naour <romain.naour at gmail.com> writes:

 > CMake < 3.11 doesn't support add_library() without any source file
 > (i.e add_library(foo SHARED)). But flann CMake use a trick that use
 > an empty string "" as source list (i.e add_library(foo SHARED "")).
 > This look like a bug in CMake < 3.11.

 > With CMake >= 3.11, the new behaviour of add_library() break the
 > existing flann CMake code.

 > From CMake Changelog [1]:
 > "add_library() and add_executable() commands can now be called without
 >  any sources and will not complain as long as sources are added later
 >  via the target_sources() command."

 > Note: flann CMake code doesn't use target_sources() since no source file
 > are provided intentionally since the flann shared library is created by
 > linking with the flann_cpp_s static library with this line:

 > target_link_libraries(flann_cpp -Wl,-whole-archive flann_cpp_s -Wl,-no-whole-archive)

 > If you try to use "add_library(flann_cpp SHARED ${CPP_SOURCES})" (as it should
 > be normally done), the link fail due to already defined symbol.

 > They are building the shared version using the static library "to speedup the
 > build time" [3]

 > This issue is already reported upstream [2] with a proposed solution.

 > Fixes:
 > http://autobuild.buildroot.net/results/b2f/b2febfaf8c44ce477b3e4a5b9b976fd25e8d7454

 > [1] https://cmake.org/cmake/help/v3.11/release/3.11.html
 > [2] https://github.com/mariusmuja/flann/issues/369
 > [3] https://github.com/mariusmuja/flann/commit/0fd62b43be2fbb0b8d791ee36290791224dc030c

 > Signed-off-by: Romain Naour <romain.naour at gmail.com>
 > Cc: Davide Viti <zinosat at tiscali.it>
 > Cc: Bernd Kuhls <bernd.kuhls at t-online.de>
 > ---
 > v2: improve the commit log (ThomasP)

Committed to 2018.02.x, thanks.

-- 
Bye, Peter Korsgaard