Fix whitespace shell bugs in configure library detection (#628)

Fix whitespace shell bugs in configure library detection (#628)

Whitespace around shell variable assign and append operators
results in those operations not being performed.
This caused the with_*, *_PATH and *_LIBS variables to not
be set to the correct path when a custom path is used.

Also, lack of condition on the *_CFLAGS assign can pollute
the compiler parameters with non-existent directories.
It should also have been an append like PATH and LIBS to
add the auto-detected value after any user-provided value
instead of overwriting.

Broken since master commit 2453140.