https://github.com/flann-lib/flann/issues/466

Index: cmake/flann.pc.in
--- cmake/flann.pc.in.orig
+++ 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} -llz4 -lflann -lflann_cpp
 Cflags: -I${includedir}
 
