Fix lz4 missing from flann.pc Requires: line.

lz4 is an unconditional dependency of flann (see #399),
but until now was not correctly generated into the
`Requires: lz4` line of `flann.pc`,
because the `PKG_EXTERNAL_DEPS` variable used in `flann.pc.in`
was not defined at all.

This fixes build error `lz4.h: No such file or directory`
for properly sandboxed builds, in which undeclared dependencies
are not made available.

CMakeLists.txt

@@ -61,6 +61,9 @@ option(USE_MPI "Use MPI" OFF)
 
 set(NVCC_COMPILER_BINDIR "" CACHE PATH  "Directory where nvcc should look for C++ compiler. This is passed to nvcc through the --compiler-bindir option.")
 
+# pkg-config dependencies to be generated into the .pc file
+set(PKGCONFIG_EXTERNAL_DEPS_LIST "")
+
 if (NOT BUILD_C_BINDINGS)
     set(BUILD_PYTHON_BINDINGS OFF)
     set(BUILD_MATLAB_BINDINGS OFF)
@@ -148,6 +151,7 @@ endif(BUILD_CUDA_LIB)
 
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(LZ4 REQUIRED liblz4)
+list(APPEND PKGCONFIG_EXTERNAL_DEPS_LIST "liblz4")
 
 #set the C/C++ include path to the "include" directory
 include_directories(BEFORE ${PROJECT_SOURCE_DIR}/src/cpp)

cmake/CMakeLists.txt

@@ -1,4 +1,13 @@
 set(PKG_DESC "Fast Library for Approximate Nearest Neighbors")
+
+# Compute `PKG_EXTERNAL_DEPS` string from `PKGCONFIG_EXTERNAL_DEPS_LIST`.
+# Once the project has `cmake_minimum_required(VERSION 2.6)`, this
+# can be replaced by `list(JOIN ...)`.
+set(PKG_EXTERNAL_DEPS "")
+foreach(_dep ${PKGCONFIG_EXTERNAL_DEPS_LIST})
+    string(APPEND PKG_EXTERNAL_DEPS " ${_dep}")
+endforeach()
+
 set(pkg_conf_file ${CMAKE_CURRENT_BINARY_DIR}/flann.pc)
 configure_file(flann.pc.in ${pkg_conf_file} @ONLY)
 install(FILES ${pkg_conf_file}