dont end up with -lm.so.X.Y in Libs.private in tiff-4.pc

Index: libtiff/CMakeLists.txt
--- libtiff/CMakeLists.txt.orig
+++ libtiff/CMakeLists.txt
@@ -181,7 +181,7 @@ if(WEBP_SUPPORT)
 endif()
 if(CMath_LIBRARY)
   target_link_libraries(tiff PRIVATE CMath::CMath)
-  list(APPEND tiff_libs_private_list "${CMath_LIBRARY}")
+  list(APPEND tiff_libs_private_list "m")
 endif()
 
 set(tiff_libs_private_list "${tiff_libs_private_list}" PARENT_SCOPE)
