Fix lz4 / HDF5 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.

Same thing for HDF5, but conditionally.

For lz4, also remove the hardcode of `@LZ4_STATIC_LDFLAGS@`
from `flann.pc.in`, as this is no longer necessary.
That fixes an incorrect `-L` flag being generated in there, e.g.
`-L/usr/lib;-llz4`. Thus fixes #480.

Author: nh2

CMakeLists.txt

@@ -62,6 +62,9 @@ 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)
@@ -81,6 +84,7 @@ if (NOT HDF5_FOUND)
 	message(WARNING "hdf5 library not found, some tests will not be run")
 else()
     include_directories(${HDF5_INCLUDE_DIR})
+    list(APPEND PKGCONFIG_EXTERNAL_DEPS_LIST "hdf5")
 endif()
 
 if (USE_MPI OR HDF5_IS_PARALLEL)
@@ -150,6 +154,10 @@ endif(BUILD_CUDA_LIB)
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(LZ4 REQUIRED liblz4)
 include_directories(${LZ4_INCLUDE_DIRS})
+# flann is exposing lz4 headers e.g. in `util/serialization.h`
+# we need lz4 to appear in `Requires` (not `Requires.private`) in the
+# `.pc` file, see https://people.freedesktop.org/~dbn/pkg-config-guide.html
+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}

cmake/flann.pc.in

@@ -8,6 +8,6 @@ Name: @PROJECT_NAME@
 Description: @PKG_DESC@
 Version: @FLANN_VERSION@
 Requires: @PKG_EXTERNAL_DEPS@
-Libs: -L${libdir} @LZ4_STATIC_LDFLAGS@ -lflann -lflann_cpp
+Libs: -L${libdir} -lflann -lflann_cpp
 Cflags: -I${includedir}