Do not treat compilation warnings as errors.

This mostly reverts commit 285a3e4fbbe8c750418c12c254de5168c010e569.
Mengyao noted that -Werror can result in unexpected compilation errors
for end-users, which may prevent them from using the library [1].

[1] https://github.com/mengyao/Complete-Striped-Smith-Waterman-Library/pull/6#issuecomment-44559156