+of options to be used for all builds. If you set this, you should
+probably set @code{CRTSTUFF_T_CFLAGS} to a dash followed by it.
+
+@findex NATIVE_SYSTEM_HEADER_DIR
+@item NATIVE_SYSTEM_HEADER_DIR
+If the default location for system headers is not @file{/usr/include},
+you must set this to the directory containing the headers. This value
+should match the value of the @code{SYSTEM_INCLUDE_DIR} macro.
+
+@findex SPECS
+@item SPECS
+Unfortunately, setting @code{MULTILIB_EXTRA_OPTS} is not enough, since
+it does not affect the build of target libraries, at least not the
+build of the default multilib. One possible work-around is to use
+@code{DRIVER_SELF_SPECS} to bring options from the @file{specs} file
+as if they had been passed in the compiler driver command line.
+However, you don't want to be adding these options after the toolchain
+is installed, so you can instead tweak the @file{specs} file that will
+be used during the toolchain build, while you still install the
+original, built-in @file{specs}. The trick is to set @code{SPECS} to
+some other filename (say @file{specs.install}), that will then be
+created out of the built-in specs, and introduce a @file{Makefile}
+rule to generate the @file{specs} file that's going to be used at
+build time out of your @file{specs.install}.
+
+@item T_CFLAGS
+These are extra flags to pass to the C compiler. They are used both
+when building GCC, and when compiling things with the just-built GCC@.
+This variable is deprecated and should not be used.