CheckStructHasMember: Do not use an empty struct for tests

Some compilers do not accept that
CMake Nightly Date Stamp
Merge topic 'CheckStructHasMember_CXX' into next

e04402f CheckStructHasMember: Do not use an empty struct for tests
Several fixes to Haiku platform module

 * Do not define BEOS anymore (this includes workarounds which we don't
	need most of the time in Haiku, so we prefer opt-in IF(HAIKU) in the
	cmake files instead).
 * On the other hand, do define UNIX (we are trying to be compliant) and
	HAIKU (there is still a number of things we don't do like the
	average UNIX clone)
 * Do not use UnixPaths, as our filesystem hierarchy isn't anything like
	what it expects.
 * Do not use -nostart, which the compiler doesn't know about anymore.
	This used to be an Haiku extension to gcc, and is equivalent to
	-shared which is the default gcc option.
 * While "dl" functions are provided in libroot, this is always
	implicitly linked so there is no need to tell cmake about it.
 * Forcing position-independent code is not needed, so remove it.
 * On the other hand, include appropriate linker options for executables
	and shared libraries.
 * Support for the two available compilers in Haiku (gcc2 and gcc4) and
	pick the right headers and libraries according to the currently
	selected one.
 * With the adoption of the package manager, the directory layout was
	changed. Tell cmake where to look for header files and libraries.
 * As we don't define BEOS anymore, enable the workaround we still need
	for HAIKU as well. This is the lack of a libm (it is part of the
	implicitly linked in libroot)
Remove use of B_COMMON_DIRECTORY

 * The common directory was removed in Haiku. Applications are now
	installed in the system directory.
Remove useless preprocessor checks

 * Haiku does not define __BEOS__ anymore, so there is no need to guard
	these BeOS specific workaround for Haiku.
 * The workaround themselves are not needed for Haiku as it has much
	better POSIX compatibility than BeOS did.
Include files cleanup

 * No need to use a different path from the BeOS one, which still works.
FindSDL: Fix broken include paths

 * I asked the CMake community about this, and they couldn't even figure
out how this worked even on other platforms.
Merge topic 'haiku-updates' into next

7eb9635 FindSDL: Fix broken include paths
209e537 Include files cleanup
5e0a4ea Remove useless preprocessor checks
144f072 Remove use of B_COMMON_DIRECTORY
fbc7c2d Several fixes to Haiku platform module
dc92741 CMake Nightly Date Stamp