Use $(error ...) instead of exit 1 to fail a Makefile.
Allow shared library suffix to be overridden
A variety of changes to SPI specific build to support OSX:
* Replace deprecated $(ARCHITECTURE) with $(SP_OS).
* Use $(SPPYARCH) more often.
* Use $(SP_OS) instead of $(SP_ARCH).
* Currently a minimal Mac OS X SpComp2 build only requires $(SP_OS).
  Since all our operating systems are 64-bit, there is no need for
  $(SP_ARCH) to distinguish between 32- and 64-bit operating systems.
* Replace $(platform) with $(SP_OS) and "macosx" with "lion".
* The SP_OS name for Mac OS X 10.7 is "lion".