diff --git a/documentation/src/unicode.dox b/documentation/src/unicode.dox
index 818d22b49..ecd9074bd 100644
--- a/documentation/src/unicode.dox
+++ b/documentation/src/unicode.dox
@@ -191,14 +191,14 @@ the following limitations:
 
 \section unicode_illegals Illegal Unicode and UTF-8 sequences
 
-Three pre-processor variables are defined in the source code that
+Three pre-processor variables are defined in the source code [1] that
 determine how %fl_utf8decode() handles illegal UTF-8 sequences:
 
 - if ERRORS_TO_CP1252 is set to 1 (the default), %fl_utf8decode() will
   assume that a byte sequence starting with a byte in the range 0x80
-  to 0x9f represents a Microsoft CP1252 character, and will instead
-  return the value of an equivalent UCS character. Otherwise, it
-  will be processed as an illegal byte value as described below.
+  to 0x9f represents a Microsoft CP1252 character, and will return
+  the value of an equivalent UCS character. Otherwise, it will be
+  processed as an illegal byte value as described below.
 
 - if STRICT_RFC3629 is set to 1 (not the default!) then UTF-8
   sequences that correspond to illegal UCS values are treated as
@@ -210,6 +210,10 @@ determine how %fl_utf8decode() handles illegal UTF-8 sequences:
   byte value is returned unchanged, otherwise 0xFFFD, the Unicode
   REPLACEMENT CHARACTER, is returned instead.
 
+[1] Since FLTK 1.3.4 you may set these three pre-processor variables on
+    your compile command line with -D"variable=value" (value: 0 or 1)
+    to avoid editing the source code.
+
 %fl_utf8encode() is less strict, and only generates the UTF-8
 sequence for 0xFFFD, the Unicode REPLACEMENT CHARACTER, if it is
 asked to encode a UCS value above U+10FFFF.
diff --git a/src/fl_utf.c b/src/fl_utf.c
index dc1ac94a5..1a5a00e57 100644
--- a/src/fl_utf.c
+++ b/src/fl_utf.c
@@ -67,7 +67,9 @@
    to completely ignore character sets in your code because virtually
    everything is either ISO-8859-1 or UTF-8.
 */
-#define ERRORS_TO_ISO8859_1 1
+#ifndef ERRORS_TO_ISO8859_1
+# define ERRORS_TO_ISO8859_1 1
+#endif
 
 /*!Set to 1 to turn bad UTF-8 bytes in the 0x80-0x9f range into the
    Unicode index for Microsoft's CP1252 character set. You should
@@ -75,7 +77,9 @@
    available text (such as all web pages) are correctly converted
    to Unicode.
 */
-#define ERRORS_TO_CP1252 1
+#ifndef ERRORS_TO_CP1252
+# define ERRORS_TO_CP1252 1
+#endif
 
 /*!A number of Unicode code points are in fact illegal and should not
    be produced by a UTF-8 converter. Turn this on will replace the
@@ -83,7 +87,9 @@
    arbitrary 16-bit data to UTF-8 and then back is not an identity,
    which will probably break a lot of software.
 */
-#define STRICT_RFC3629 0
+#ifndef STRICT_RFC3629
+# define STRICT_RFC3629 0
+#endif
 
 #if ERRORS_TO_CP1252
 /* Codes 0x80..0x9f from the Microsoft CP1252 character set, translated
@@ -103,7 +109,7 @@ static unsigned short cp1252[32] = {
     (adding \e len to \e p will point at the next character).
 
     If \p p points at an illegal UTF-8 encoding, including one that
-    would go past \e end, or where a code is uses more bytes than
+    would go past \e end, or where a code uses more bytes than
     necessary, then *(unsigned char*)p is translated as though it is
     in the Microsoft CP1252 character set and \e len is set to 1.
     Treating errors this way allows this to decode almost any