Minor toolchain fixes

[GriffinRichards]

• Fix a bug with [Build System Rewrite] Massive build speed improvement via scaninc changes #1954. If generating the .o failed due to a non-existent dependency, the .d file wasn't regenerated (e.g. for some #include "foo.h", if the build failed because foo.h didn't exist then removing the #include wouldn't allow the build to proceed)
• Clarify the error message when a dependency file doesn't exist by allowing scaninc to ignore it (example below)
• Silence some warnings when a .d file doesn't exist yet
• Silence some warnings for -Wunqualified-std-cast-call
• Explicitly default KEEP_TEMPS to 0

Example -- given #include "foo.h" in parent.c where foo.h doesn't exist...
before:

make: *** No rule to make target `src/foo.h', needed by `build/emerald/src/parent.o'.  Stop.

after:

src/parent.c:<line_num>:<col_num>: fatal error: foo.h: No such file or directory
    <line_num> | #include "foo.h"