check for definition of JSON_IS_AMALGAMATION before setting it to avoid compiler warning of redefintion.

Author: mattpolzin

amalgamate.py

@@ -65,7 +65,9 @@ def amalgamate_source(source_top_dir=None,
     header.add_text("# define JSON_AMALGATED_H_INCLUDED")
     header.add_text("/// If defined, indicates that the source file is amalgated")
     header.add_text("/// to prevent private header inclusion.")
+    header.add_text('#ifndef JSON_IS_AMALGAMATION')
     header.add_text("#define JSON_IS_AMALGAMATION")
+    header.add_text('#endif')
     header.add_file("include/json/version.h")
     header.add_file("include/json/config.h")
     header.add_file("include/json/forwards.h")
@@ -92,7 +94,9 @@ def amalgamate_source(source_top_dir=None,
     header.add_text("# define JSON_FORWARD_AMALGATED_H_INCLUDED")
     header.add_text("/// If defined, indicates that the source file is amalgated")
     header.add_text("/// to prevent private header inclusion.")
+    header.add_text('#ifndef JSON_IS_AMALGAMATION')
     header.add_text("#define JSON_IS_AMALGAMATION")
+    header.add_text('#endif')
     header.add_file("include/json/config.h")
     header.add_file("include/json/forwards.h")
     header.add_text("#endif //ifndef JSON_FORWARD_AMALGATED_H_INCLUDED")