build: allow non-standard variations of linker -l/-L flags

build: allow non-standard variations of linker -l/-L flags

This enables replacing the -l and -L flags used to specify the
just-built libraries when linking the tools and shared libs with
non-standard syntaxes.  System library flags are already handled
by the filtering mechanism in configure.

Signed-off-by: Mans Rullgard <mans@mansr.com>