thttpd: fix build with GCC >= 14

Unfortunately there is no way in the configure script to persist CFLAGS
from configure to the final build (CFLAGS is respected by configure, but
by configure only). This roughly matches what alpine is doing now: They
pass -fpermissive to configure via CFLAGS to avoid -Wimplicit-int, the
-Wimplicit-function-declaration in the actual build seemingly never
happens on Alpine, probably due to musl.

pkgs/by-name/th/thttpd/package.nix

···
     sed -i -e 's/chmod 2755/chmod 755/' extras/Makefile.in
   '';
 
+  # Work around failures with GCC 14, upstream is inactive unfortunately
+  # https://gcc.gnu.org/gcc-14/porting_to.html#warnings-as-errors
+  NIX_CFLAGS_COMPILE = [
+    "-Wno-error=implicit-int"
+    "-Wno-error=implicit-function-declaration"
+  ];
+
   buildInputs = [
     libxcrypt
   ];