Fix typo in MANUAL.md

Correct 'make us' to 'make use'.

For unity targets, the PREFIX and SUFFIX target properties were being
copied so that library output file names would make the non-unity
target.  However, this wasn't the case for IMPORT_PREFIX and
IMPORT_SUFFIX which control the import library file name on Windows.  As
a result, the unity target and the original target differed in the
output file name for import libraries.

This fix simply adds IMPORT_PREFIX and IMPORT_SUFFIX to the list of
properties being copied from the original target.

Fix `-isystem` includes being incorrectly passed to execute_process()

The external command was being invoked with arguments: ["clang", ...,
"-isystem  /foo/bar/baz", ...], which meant that the compiler treated
the whitespace between the flag and the path as being part of the path
itself. (i.e. it was looking for "  /foo/bar/baz").

Fixed by passing the argument to execute_process() as
'-isystem/foo/bar/baz'; I also needed to ensure we normalized the value
of CMAKE_INCLUDE_SYSTEM_FLAG_<LANG>, which itself contains trailing
whitespace in some cases.

Update a link to problems with unity builds

Lee Winder have moved hist blog to the new place, old link doesn't work now, but I believe I have found the correct one

Check for empty architectures before setting -arch flag.

Preserve system flag for includes when generating PCH

If the compiler understands system include paths it exposes the
necessary flag within the CMAKE_INCLUDE_SYSTEM_FLAG_${LANG}
variable.

System include paths for targets could be queried via the target
property INTERFACE_SYSTEM_INCLUDE_DIRECTORIES. Here the system
include paths are listed in addition of being available in
the INTERFACE_INCLUDE_DIRECTORIES property.

Note: Currently there is no way to query the SYSTEM flag for
directory include properties.

Check for empty variable