getpagesize: Fix use of 'extern inline' (regression 2012-11-20). * lib/unistd.in.h (_GL_GETPAGESIZE_INLINE): New macro. (getpagesize): Use it. * lib/getpagesize.c (_GL_GETPAGESIZE_INLINE): Define. * modules/getpagesize (configure.ac): Compile getpagesize.c also when getpagesize is missing.