configure: fix $prefix expansion for libdir_expanded

configure: fix $prefix expansion for libdir_expanded

Since the sed expansion line which did $prefix expansion for
libdir_expanded was changed from the legacy `...` format to the
new $(...) expression, the subtle backslash expansion change went
unnoticed -- \\$ which used to escape '$' now escapes '\' and the
following '$prefix' was attempted to expand as a variable. So
changing \\$ to \$ fixes this.

Also, replaced echo with printf %s -- echo does expansions of its own.

While at it, the following 2 inconsistencies were fixed:
 1) the /g flag was removed from first expression; second didn't have it
 2) first expression did not end with /, so "dropped" it from second

 configure | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)