Description
The new compiler option '-fx' (to enable all warnings suppressed by default) is not yet supported in SDT 2.50. It was correctly documented in the SDT 2.50 Reference Guide, but then unfortunately never enabled in the compiler.
This is fixed in SDT 2.51 and C++ 1.11

The compile option '-fd', which causes the compiler to look for double-quoted include files (e.g #include "header.h") in the same place (same search path) as for angle-brace include files (e.g #include <header.h>), was accidentally omitted from armcc/tcc (in SDT 2.50) and armcpp/tcpp (in C++ 1.10).
This is fixed in SDT 2.51 and C++ 1.11

Compiling with this option results in the following message:

"<command line>", line 1: Warning: unknown option -fd: ignored 

'-fd' is however still documented in Table 2-1 of the SDT 2.50 Reference Guide.

Solution (for SDT 2.50 & C++ 1.10)
As a workaround for '-fd', you can create the same effect by using '< >' quotes in includes instead of "" for the names of the header files.

#if __ARMCC_VERSION == 4.90
#include <header.h>
#else
#include "header.h"
#endif 

Alternatively, it may be possible to use the '-fk' option, which ensures that the 'current place' always remains the directory the C file is compiled in.

See also the (default) '-Wp' compiler option which suppresses warnings for #include <...> of non-ANSI header files, as described on page 2-31 of the SDT 2.50 Reference Guide.