From https://src.fedoraproject.org/rpms/lpcnetfreedv/blob/rawhide/f/lpcnetfreedv-libm.patch

Index: LPCNet-0.5/src/CMakeLists.txt
===================================================================
--- LPCNet-0.5.orig/src/CMakeLists.txt
+++ LPCNet-0.5/src/CMakeLists.txt
@@ -42,6 +42,10 @@ endif(LPCNET_C_PROC_FLAGS)
 
 add_library(lpcnetfreedv SHARED ${lpcnet_freedv_srcs} ${codec2_import_srcs})
 
+if(UNIX)
+target_link_libraries(lpcnetfreedv m)
+endif()
+
 set_target_properties(lpcnetfreedv PROPERTIES
     PUBLIC_HEADER lpcnet_freedv.h
 	VERSION ${LPCNET_VERSION}