Compile programs with -D_POSIX_C_SOURCE=200809L
Compile programs with -D_POSIX_C_SOURCE=200809L

The _DEFAULT_SOURCE feature test macro is only supported by glibc 2.19
and later.  As a result, various things were not being defined when
building with an older glibc version, causing compile errors.  Instead,
_POSIX_C_SOURCE=200809L should expose everything we need.

ebiggers/libdeflate,ebiggers/libdeflate,ebiggers/libdeflate