Fix:

error: use of undeclared identifier 'FALSE'

https://github.com/kisli/vmime/issues/247
https://github.com/kisli/vmime/commit/ec5b8f7cfe38e58b38eb5613b63b4b8f1a8343e0
--- src/vmime/charsetConverter_icu.cpp.orig	2023-08-18 17:47:14.000000000 -0500
+++ src/vmime/charsetConverter_icu.cpp	2023-08-18 17:48:50.000000000 -0500
@@ -359,7 +359,7 @@
 		toErr = U_ZERO_ERROR;
 
 		ucnv_toUnicode(m_from, &uniTarget, uniTargetLimit,
-		               &uniSource, uniSourceLimit, NULL, /* flush */ FALSE, &toErr);
+		               &uniSource, uniSourceLimit, NULL, /* flush */ UBool(0), &toErr);
 
 		if (U_FAILURE(toErr) && toErr != U_BUFFER_OVERFLOW_ERROR)
 		{
@@ -396,7 +396,7 @@
 			fromErr = U_ZERO_ERROR;
 
 			ucnv_fromUnicode(m_to, &cpTarget, cpTargetLimit,
-							 &cpSource, cpSourceLimit, NULL, /* flush */ FALSE, &fromErr);
+							 &cpSource, cpSourceLimit, NULL, /* flush */ UBool(0), &fromErr);
 
 			if (fromErr != U_BUFFER_OVERFLOW_ERROR && U_FAILURE(fromErr))
 			{
@@ -448,7 +448,7 @@
 		toErr = U_ZERO_ERROR;
 
 		ucnv_toUnicode(m_from, &uniTarget, uniTargetLimit,
-		               &uniSource, uniSourceLimit, NULL, /* flush */ TRUE, &toErr);
+		               &uniSource, uniSourceLimit, NULL, /* flush */ UBool(1), &toErr);
 
 		if (U_FAILURE(toErr) && toErr != U_BUFFER_OVERFLOW_ERROR)
 		{
@@ -476,7 +476,7 @@
 			fromErr = U_ZERO_ERROR;
 
 			ucnv_fromUnicode(m_to, &cpTarget, cpTargetLimit,
-							 &cpSource, cpSourceLimit, NULL, /* flush */ TRUE, &fromErr);
+							 &cpSource, cpSourceLimit, NULL, /* flush */ UBool(1), &fromErr);
 
 			if (fromErr != U_BUFFER_OVERFLOW_ERROR && U_FAILURE(fromErr))
 			{