Bug#897101: flann FTBFS with cmake 3.11.1

[Adrian Bunk]

Source: flann
Version: 1.9.1+dfsg-2
Severity: serious
Tags: buster sid

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/flann.html

...
-- Configuring done
CMake Error at src/cpp/CMakeLists.txt:32 (add_library):
  No SOURCES given to target: flann_cpp


CMake Error at src/cpp/CMakeLists.txt:86 (add_library):
  No SOURCES given to target: flann