From:
https://llvm.org/bugs/show_bug.cgi?id=13237

"clang mirrors gcc in that it allows an optional space when specifying include
paths, i.e. it supports both `-I/DIR and `-I /DIR` or along the same line
`-isystem/DIR` and `-isystem /DIR`.

scan-build understands most of these as well, but not the `-isystem/DIR` form
without the extra space."

a simple patch to scan-build will allow for -isystem arguments

patch from:
Thomas Hauth
https://llvm.org/bugs/attachment.cgi?id=13705

not committed yet, and it still present in all versions of clang. This is
especially a problem using cmake as it uses -isystem for system libraries by
default.