MAINT: add src/private in build_dir to include search paths

allows generating headers like npy_partition.h with the code generator
ENH: add scalarmathmodule.h.src with integer overflow functions

and use them in scalarmathmodule.c instead of the old (disabled) ones.
MAINT: add library sources to some dependencies

fixes missing recompilation of parts of numpy when the static libraries
change.
Merge pull request #3917 from juliantaylor/scalar-int-overflow

check integer overflows in scalarmath