Fixes this issue:
 <https://github.com/mariusmuja/flann/issues/369>.
Patch from Buildroot:
 <https://github.com/buildroot/buildroot/commit/45a39b3e2ba42b72d19bfcef30db1b8da9ead51a>.

From fa5ec96a94646492a3f908e12905b3e48a8e800b Mon Sep 17 00:00:00 2001
From: Romain Naour <romain.naour@gmail.com>
Date: Wed, 18 Apr 2018 20:24:13 +0200
Subject: [PATCH] src/cpp: fix cmake >= 3.11 build

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.

Upstream status: Pending

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@gmail.com>
---
 src/cpp/CMakeLists.txt | 4 ++--
 src/cpp/empty.cpp      | 1 +
 2 files changed, 3 insertions(+), 2 deletions(-)
 create mode 100644 src/cpp/empty.cpp

diff --git a/src/cpp/CMakeLists.txt b/src/cpp/CMakeLists.txt
index b44a735..a816863 100644
--- a/src/cpp/CMakeLists.txt
+++ b/src/cpp/CMakeLists.txt
@@ -29,7 +29,7 @@ if (BUILD_CUDA_LIB)
 endif()
 
 if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND CMAKE_COMPILER_IS_GNUCC)
-    add_library(flann_cpp SHARED "")
+    add_library(flann_cpp SHARED "empty.cpp")
     set_target_properties(flann_cpp PROPERTIES LINKER_LANGUAGE CXX)
     target_link_libraries(flann_cpp -Wl,-whole-archive flann_cpp_s -Wl,-no-whole-archive)
 
@@ -85,7 +85,7 @@ if (BUILD_C_BINDINGS)
     set_property(TARGET flann_s PROPERTY COMPILE_DEFINITIONS FLANN_STATIC)
 
     if(CMAKE_SYSTEM_NAME STREQUAL "Linux" AND CMAKE_COMPILER_IS_GNUCC)
-        add_library(flann SHARED "")
+        add_library(flann SHARED "empty.cpp")
         set_target_properties(flann PROPERTIES LINKER_LANGUAGE CXX)
         target_link_libraries(flann -Wl,-whole-archive flann_s -Wl,-no-whole-archive)
     else()
diff --git a/src/cpp/empty.cpp b/src/cpp/empty.cpp
new file mode 100644
index 0000000..40a8c17
--- /dev/null
+++ b/src/cpp/empty.cpp
@@ -0,0 +1 @@
+/* empty */
-- 
2.14.3
