CMake: Don't assume that -fvisibility=hidden is supported outside Windows.

The original code was good enough for supporting GNU/Linux
and a few others but it wasn't very portable.

CMake doesn't support Solaris Studio's -xldscope=hidden.
If it ever does, things should still work with this commit
as Solaris Studio supports not only its own __global but also
the GNU C __attribute__((visibility("default"))). Support for the
attribute was added in 2007 to Sun Studio 12 compiler version 5.9.

CMakeLists.txt

@@ -856,10 +856,24 @@ calculation if supported by the system" ON)
     endif()
 endif()
 
-# Support -fvisiblity=hidden when building shared liblzma.
+# Symbol visibility support:
-# These lines do nothing on Windows (even under Cygwin).
+#
-# HAVE_VISIBILITY should always be defined to 0 or 1.
+# The C_VISIBILITY_PRESET property takes care of adding the compiler
-if(BUILD_SHARED_LIBS)
+# option -fvisibility=hidden (or equivalent) if and only if it is supported.
+#
+# HAVE_VISIBILITY indicates if __attribute__((__visibility__("default")))
+# is supported. HAVE_VISIBILITY is ignored on Windows and Cygwin in
+# the C code so we don't need to handle that here. HAVE_VISIBILITY
+# should always be defined to 0 or 1.
+#
+# CMake's GenerateExportHeader module is too fancy since liblzma already
+# has the necessary macros. Instead, check CMake's internal variable
+# CMAKE_C_COMPILE_OPTIONS_VISIBILITY (it's the C-specific variant of
+# CMAKE_<LANG>_COMPILE_OPTIONS_VISIBILITY) which contains the compiler
+# command line option for visibility support. It's empty or unset when
+# visibility isn't supported. It was added to CMake 2.8.12 in the commit
+# 0e9f4bc00c6b26f254e74063e4026ac33b786513 in 2013.
+if(BUILD_SHARED_LIBS AND CMAKE_C_COMPILE_OPTIONS_VISIBILITY)
     set_target_properties(liblzma PROPERTIES C_VISIBILITY_PRESET hidden)
     target_compile_definitions(liblzma PRIVATE HAVE_VISIBILITY=1)
 else()