+# b4_define_flag_if(FLAG)
+# -----------------------
+# Define "b4_FLAG_if(IF-TRUE, IF-FALSE)" that depends on the
+# value of the Boolean FLAG.
+m4_define([b4_define_flag_if],
+[_b4_define_flag_if($[1], $[2], [$1])])
+
+# _b4_define_flag_if($1, $2, FLAG)
+# --------------------------------
+# This macro works around the impossibility to define macros
+# inside macros, because issuing `[$1]' is not possible in M4 :(.
+# This sucks hard, GNU M4 should really provide M5 like $$1.
+m4_define([_b4_define_flag_if],
+[m4_if([$1$2], $[1]$[2], [],
+ [m4_fatal([$0: Invalid arguments: $@])])dnl
+m4_define([b4_$3_if],
+ [b4_flag_if([$3], [$1], [$2])])])
+
+
+# b4_FLAG_if(IF-TRUE, IF-FALSE)