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()