Fix triple-letter typos in comments and documentation

* ChangeLog: Fix triple-letter typos.
* doc/overview.html: Likewise.
* mark.c (GC_push_marked1): Fix a triple-letter typo in a comment.
* os_dep.c (GC_register_data_segments): Likewise.
Remove GC_BUILD definition from build scripts

* CMakeLists.txt (ADD_DEFINITIONS): Remove -D GC_BUILD (not
needed since it is defined in gc_priv.h, gc_pmark.h and
gc_cpp.cc).
* WCC_MAKEFILE (CFLAGS): Likewise.
* build/s60v3/libgc.mmp: Likewise.
* digimars.mak (DEFINES, tests/test.obj): Likewise.
* windows-untested/vc60/gc.dsp (ADD CPP): Likewise.
* windows-untested/vc60/libgc.dsp (ADD CPP): Likewise.
* windows-untested/vc60/libgcmt.dsp (ADD CPP): Likewise.
* windows-untested/vc70/gc.vcproj (PreprocessorDefinitions): Likewise.
* windows-untested/vc70/libgc.vcproj (PreprocessorDefinitions):
Likewise.
* windows-untested/vc70/libgcmt.vcproj (PreprocessorDefinitions):
Likewise.
* windows-untested/vc71/gc.vcproj (PreprocessorDefinitions): Likewise.
* windows-untested/vc71/libgc.vcproj (PreprocessorDefinitions):
Likewise.
* windows-untested/vc71/libgcmt.vcproj (PreprocessorDefinitions):
Likewise.
* extra/MacOS.c: Define GC_BUILD (before include of gc.h).
* extra/msvc_dbg.c: Likewise.
* windows-untested/vc60/gc.dsp (MESSAGE, ENDIF): Remove trailing
spaces at EOLn.
* windows-untested/vc60/libgc.dsp (MESSAGE, ENDIF): Likewise.
* windows-untested/vc60/libgcmt.dsp (MESSAGE, ENDIF): Likewise.
Enable staticrootstest for the case of GC shared library build

* tests/staticrootslib.c (GC_TEST_EXPORT_API): New macro (defined to
set GCC default visibility attribute if GC_VISIBILITY_HIDDEN_SET
unless Win32).
* tests/staticrootslib.c (root): Remove unused global variable.
* tests/staticrootslib.c (libsrl_mktree, libsrl_init): Decorate with
GC_TEST_EXPORT_API.
* tests/staticrootstest.c (GC_VISIBILITY_HIDDEN_SET): Do not check (do
not skip the test).
* tests/staticrootstest.c (GC_TEST_IMPORT_API): New macro.
* tests/staticrootstest.c (root): Explicitly specify zero initializer.
* tests/staticrootstest.c (libsrl_mktree, libsrl_init): Decorate with
GC_TEST_IMPORT_API.
* tests/staticrootstest.c (mktree, main): Remove commented out code.
* tests/staticrootstest.c (main): Avoid code duplication.
* tests/tests.am (staticrootstest_LDADD, check_LTLIBRARIES,
libstaticrootslib_la): Rename libstaticrootslib to
libstaticrootslib_test.
Improve staticrootstest: add global data to library, add lib w/o GC_INIT

* tests/staticrootslib.c: Do not include stdio.h (not needed).
* tests/staticrootslib.c (root, root_nz): New static variables.
* tests/staticrootstest.c (root_nz): Likewise.
* tests/staticrootslib.c (libsrl_getpelem): New exported function
(exported as "libsrl_getpelem2" if STATICROOTSLIB2).
* tests/staticrootslib.c (libsrl_mktree, libsrl_init): Do not define
if STATICROOTSLIB2.
* tests/staticrootslib.c (libsrl_init): Do not call GC_INIT if
STATICROOTSLIB_INIT_IN_MAIN defined.
* tests/staticrootstest.c (libsrl_getpelem, libsrl_getpelem2): Declare
imported function.
* tests/staticrootstest.c (main): Call GC_INIT if
STATICROOTSLIB_INIT_IN_MAIN defined; set *libsrl_getpelem2() (only if
STATICROOTSLIB2 defined), *libsrl_getpelem() and root_nz[] elements
(similar to that of root[]).
Adjust build scripts to enable additional test library in staticrootstest
(The 2nd test library contains global data too but has no GC_INIT call.)

* tests/tests.am (staticrootstest_CFLAGS,
libstaticrootslib2_test_la_CFLAGS): New definition (to test with
libsrl_getpelem2() enabled).
* tests/tests.am (staticrootstest_LDADD, check_LTLIBRARIES): Add
libstaticrootslib2_test.la.
* tests/tests.am (libstaticrootslib2_test_la_SOURCES,
libstaticrootslib2_test_la_LIBADD,
libstaticrootslib2_test_la_LDFLAGS): New definition (similar to that
of libstaticrootslib_test_la_X).
Convert readme to markdown.
Adjust README filename in README.QUICK
Adjust README.md title and references to doc .html files in it
Default to use libc_stack_end in single-threaded GC on glibc targets

* include/private/gcconfig.h (USE_LIBC_PRIVATES): Define for glibc
(unless DONT_USE_LIBC_PRIVATES is defined).