Fix pthread_start compilation if single-obj-compilation (Linux)

Fix pthread_start compilation if single-obj-compilation (Linux)
(Cherry-pick commits 8240ed0, b45d96b from 'master' branch.)

* Makefile.am [SINGLE_GC_OBJ && PTHREAD_START_STANDALONE] (AM_CPPFLAGS):
Add -DGC_PTHREAD_START_STANDALONE.
* Makefile.am [SINGLE_GC_OBJ && PTHREAD_START_STANDALONE]
(libgc_la_SOURCES): Add pthread_start.c entry.
* Makefile.am: Add bank lines (to improve readability).
* configure.ac (pthread_start_standalone): New variable.
* configure.ac [*-*-*linux* && THREADS=posix] (pthread_start_standalone):
Set to yes (instead of AM_CONDITIONAL(PTHREAD_START_STANDALONE) directly).
* configure.ac (PTHREAD_START_STANDALONE): AM_CONDITIONAL if
pthread_start_standalone.