-Werror,-Wundef

For MSVC/clang-cl: "__STDC__: Indicates conformance with the ANSI/ISO C99
tandard. Defined as the integer literal constant 1 only if the /Za compiler
option is given and you are not compiling C++ code; otherwise is undefined."
(<https://msdn.microsoft.com/en-us/library/b0084kay.aspx>)

Presumably fine to just replace #if with #ifdef here (as is used in all the
other cases surrounding it), and not care whether it actually expands to true.

Change-Id: Ie58c76d7e72321b06a11a370fd28221e85fcf871

soltools/mkdepend/imakemdep.h

@@ -566,7 +566,7 @@ struct pair predefs[] = {
 #ifdef __GNUC__
     {"__GNUC__", "1", NULL},
 #endif
-#if __STDC__
+#ifdef __STDC__
     {"__STDC__", "1", NULL},
 #endif
 #ifdef __HIGHC__