filemem: fix fwrite() return check in fm_write();

test `ferror()` also in `fm_err()` if `err` zero and file
NetBSD: add instructions and some workarounds (`getopt_long_only()`
  in particular)
qzint: check `__GNUC__` version for "-Wstringop-truncation"
  suppression

README.bsd

@@ -1,4 +1,4 @@
-% Tested on FreeBSD 13.2-RELEASE (with X11 + GNOME installed) and OpenBSD 7.3 (with X11)
+% Tested on FreeBSD 14.0-RELEASE (with X11 + GNOME installed), OpenBSD 7.4 (with X11) and NetBSD 9.3 (with X11)
 
 1. Prerequisites for building zint
 ==================================
@@ -15,6 +15,12 @@ or OpenBSD (make and clang should already be installed):
     pkg_add git cmake png
     exit
 
+or NetBSD (make and gcc should already be installed):
+
+    su
+    pkgin install git cmake png
+    exit
+
 Then clone the latest source
 
     git clone https://git.code.sf.net/p/zint/code zint
@@ -36,6 +42,12 @@ On OpenBSD:
     pkg_add qtbase qttools qtsvg
     exit
 
+On NetBSD:
+
+    su
+    pkgin install qt5-qtbase qt5-qttools qt5-qtsvg
+    exit
+
 
 3. Build
 ========
@@ -54,6 +66,10 @@ except that on OpenBSD you need to use
 
     cmake -DCMAKE_PREFIX_PATH=/usr/local/lib/qt5/cmake ..
 
+and on NetBSD
+
+    cmake -DCMAKE_PREFIX_PATH=/usr/pkg/qt5 ..
+
 instead.