configure: only strip first -L from LDFLAGS

configure: only strip first -L from LDFLAGS

In the logic that works out if a given OpenSSL path works, it stripped
off a possibly leading -L flag using an incorrect sed pattern which
would remove all instances of -L in the string, including if the path
itself contained that two-letter sequence!

The same pattern was used and is now updated in multiple places. Now it
only removes -L if it starts the strings.

Reported-by: Mohamed Osama
Fixes #5519
Closes #5521