additional fix for building with ICU 76.1 (B3_0_Release)

There is another define needed to avoid unused C++ code that otherwise requires -std=c++17

Author: real-dam

builds/posix/make.defaults

@@ -102,7 +102,7 @@ GLOB_OPTIONS:=
 #____________________________________________________________________________
 
 # Firebird needs no RTTI, make it build with icu-75.1
-RTTI_FLAG:= -fno-rtti -DU_SHOW_CPLUSPLUS_API=0
+RTTI_FLAG:= -fno-rtti -DU_SHOW_CPLUSPLUS_API=0 -DU_SHOW_CPLUSPLUS_HEADER_API=0
 
 # If this is defined then we use special rules useful for developers only
 IsDeveloper = @DEVEL_FLG@