runtime(c): Add missing syntax test files

These were missing from commit c2a967a.

closes: #16470

Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/syntax/testdir/input/c_preprocessor_conditional.c

@@ -0,0 +1,40 @@
+// C preprocessor - conditional inclusion
+
+// Source: https://en.cppreference.com/w/c/preprocessor/conditional
+
+#define ABCD 2
+#include <stdio.h>
+
+int main(void)
+{
+
+#ifdef ABCD
+    printf("1: yes\n");
+#else
+    printf("1: no\n");
+#endif
+
+#ifndef ABCD
+    printf("2: no1\n");
+#elif ABCD == 2
+    printf("2: yes\n");
+#else
+    printf("2: no2\n");
+#endif
+
+#if !defined(DCBA) && (ABCD < 2 * 4 - 3)
+    printf("3: yes\n");
+#endif
+
+// C23 directives #elifdef/#elifndef
+#ifdef CPU
+    printf("4: no1\n");
+#elifdef GPU
+    printf("4: no2\n");
+#elifndef RAM
+    printf("4: yes\n"); // selected in C23 mode, may be selected in pre-C23 mode
+#else
+    printf("4: no3\n"); // may be selected in pre-C23 mode
+#endif
+}
+