Convert GCC 14 errors back to warnings (#1247)

https://gcc.gnu.org/gcc-14/porting_to.html#warnings-as-errors

Changing these to warnings helps build code with `cosmocc`. Perhaps this
can be a patch to `cosmocc` or skipped entirely.

Author: ahgamut

libc/integral/c.inc

@@ -6,6 +6,12 @@
 #define COSMOPOLITAN_CXX_USING_
 #endif
 
+#ifndef __cplusplus
+#pragma GCC diagnostic warning "-Wimplicit-function-declaration"
+#pragma GCC diagnostic warning "-Wincompatible-pointer-types"
+#pragma GCC diagnostic warning "-Wint-conversion"
+#endif
+
 #if !defined(__GNUC__) && __cplusplus + 0 >= 201103L
 #define typeof(x) decltype(x)
 #elif !defined(__GNUC__) && __STDC_VERSION__ + 0 < 201112