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.