make assertions opt-in (#447)

slight addendum to 657080a7e5

instead of disabling asserts by adding -DNDEBUG to config.mk, this
disables asserts by default in the source code itself. this way, if
someone compiles with `make CFLAGS="-O3 -march=native"` without knowing
about asserts/-DNDEBUG then he won't accidentally get a build with
assertions in it.

this basically makes the assertions opt-in, if someone wants it, he'll
need to *explicitly* set `-DDEBUG` to get it. so that it's not possible
to accidentally end up with assertions enabled.

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/447
Reviewed-by: TAAPArthur <taaparthur@noreply.codeberg.org>

config.mk

@@ -26,7 +26,7 @@ CC = c99
 # CFLAGS, any additional compiler flags goes here
 CFLAGS = -Wall -pedantic -O2 -DNDEBUG
 # Uncomment for a debug build using gcc/clang
-# CFLAGS = -Wall -pedantic -g3 -fsanitize=address,undefined
+# CFLAGS = -Wall -pedantic -DDEBUG -g3 -fsanitize=address,undefined
 # LDFLAGS = $(CFLAGS)
 
 # icons that will be installed via `make icon`

nsxiv.h

@@ -20,6 +20,10 @@
 #ifndef NSXIV_H
 #define NSXIV_H
 
+#if !defined(DEBUG) && !defined(NDEBUG)
+	#define NDEBUG
+#endif
+
 #include <stdbool.h>
 #include <stddef.h>