add --search-path-linker option to control linker options at build time

[lexming]

Follow-up to #4645.

Add option --search-path-linker to control how linker options are handled at build time

• same approach used as for --search-path-cpp-headers
• default behaviour does not change, we continue to use LDFLAGS
• added option to use LIBRARY_PATH instead

[lexming]

@Micket this should be easy to merge now that --search-path-cpp-headers is already merged

[bartoldeman]

lgtm