Set SUBDIRS equal to autoconf variable RTLIBSUBSYS

Otherwise, the make "dist" target fails if Gambit Universal IDE
(guide) is disabled by configure.
Add make "doc" as prerequisite to make "dist" and "dist-devel" targets

Otherwise, make "dist" and make "dist-devel" fails due to missing
documentation deliverables.
Add autoconf variable RTLIBMAKEFILES

Otherwise, configure fails for distributions created by the make
"dist-devel" target if Gambit Universal IDE (guide) is disabled by the
initial configure step.
Update *.c files
Updates for iOS 6.0