Ban building Nix with NDEBUG

When reviewing old PRs, I found that #9997 adds some code to ensure one
particular assert is always present. But, removing asserts isn't
something we do in our own release builds either in the flake here or in
nixpkgs, and is plainly a bad idea that increases support burden,
especially if other distros make bad choices of build flags in their Nix
packaging.

For context, the assert macro in the C standard is defined to do nothing
if NDEBUG is set.

There is no way in our build system to set -DNDEBUG without manually
adding it to CFLAGS, so this is simply a configuration we do not use.
Let's ban it at compile time.

I put this preprocessor directive in src/libutil.cc because it is not
obvious where else to put it, and it seems like the most logical file
since you are not getting a usable nix without it.

src/libutil/util.cc

@@ -9,6 +9,10 @@
 
 #include <sodium.h>
 
+#ifdef NDEBUG
+#error "Nix may not be built with assertions disabled (i.e. with -DNDEBUG)."
+#endif
+
 namespace nix {
 
 void initLibUtil() {

tests/unit/libstore/outputs-spec.cc

@@ -6,11 +6,9 @@
 
 namespace nix {
 
-#ifndef NDEBUG
 TEST(OutputsSpec, no_empty_names) {
     ASSERT_DEATH(OutputsSpec::Names { std::set<std::string> { } }, "");
 }
-#endif
 
 #define TEST_DONT_PARSE(NAME, STR)           \
     TEST(OutputsSpec, bad_ ## NAME) {        \