modtool: removing RTLD_GLOBAL hack for swig in gr-modtool.

This was removed from GNU Radio a while ago due to newer issues. Looks like it's affecting OOT modules as well.
volk: add check for posix_memalign for OSs that do not directly set _XOPEN_SOURCE or _POSIX_C_SOURCE; define and use the new variable when found.
volk: simplifies and fixes thread safety issue in volk_malloc/free.
blocks: delay block cannot be initialized with a negative number.

This commit puts in a check in the constructor and better
documentation about the behavior.