Fix -Werror=return-type errors.

See also https://bugs.gentoo.org/902483

--- a/converter/other/pngx.c
+++ b/converter/other/pngx.c
@@ -368,6 +368,7 @@
     case PNGX_ABSOLUTE_COLORIMETRIC: return "ABSOLUTE_COLORIMETRIC";
     }
     assert(false);
+    abort();
 }
 
 
@@ -383,6 +384,7 @@
     }
 
     assert(false);  /* All cases above return */
+    abort();
 }
 
 
--- a/editor/pamcat.c
+++ b/editor/pamcat.c
@@ -1211,6 +1211,7 @@
     case JUST_CENTER: return (outpamP->width - inpamP->width) / 2;
     }
     assert(false);
+    abort();
 }
 
 
--- a/lib/libpnm3.c
+++ b/lib/libpnm3.c
@@ -433,6 +433,7 @@
     case PBM_WHITE: return pnm_whitexel(maxval, PBM_TYPE); break;
     default:
         assert(false);
+        abort();
     }
 }
 
--- a/lib/pm.h
+++ b/lib/pm.h
@@ -94,6 +94,15 @@
   #define PM_S_IRUSR S_IRUSR
 #endif
 
+/* define NORETURN macro to fix -Werror=return-type */
+#if __STDC_VERSION__ > 202300L
+#define NORETURN [[noreturn]]
+#elif __STDC_VERSION__ >= 201112L
+#define NORETURN _Noreturn
+#else
+#define NORETURN
+#endif
+
 
 
 typedef struct {
@@ -181,7 +190,7 @@
 pm_setjmpbufsave(jmp_buf *  const jmpbufP,
                  jmp_buf ** const oldJmpbufPP);
 
-void
+void NORETURN
 pm_longjmp(void);
 
 void
@@ -216,7 +225,7 @@
 void PM_GNU_PRINTF_ATTR(1,2)
 pm_errormsg(const char format[], ...);
 
-void PM_GNU_PRINTF_ATTR(1,2)
+void PM_GNU_PRINTF_ATTR(1,2) NORETURN
 pm_error (const char reason[], ...);
 
 int
--- a/lib/libpm.c
+++ b/lib/libpm.c
@@ -91,7 +91,7 @@
 
 
 
-void
+void NORETURN
 pm_longjmp(void) {
 
     if (pm_jmpbufP)
@@ -258,7 +258,7 @@
 
 
 
-void PM_GNU_PRINTF_ATTR(1,2)
+void PM_GNU_PRINTF_ATTR(1,2) NORETURN
 pm_error(const char format[], ...) {
     va_list args;
     const char * msg;