Repaired amalgamation script

The amalgamation script now searches for files in the different
directories of src.

Also the full paths in the amalgamation were replaced by the basename
to produce the same content on all machines.

The makefile now creates the build directory for the amalgamation
if it does not exist.

util/generate_amalgamation.py

@@ -1,7 +1,7 @@
 #!/usr/bin/env python
 
 import sys
-from os.path import basename, dirname, join, realpath
+from os.path import basename, dirname, join, realpath, isfile
 from glob import iglob
 import re
 
@@ -12,6 +12,18 @@ WREN_DIR = dirname(dirname(realpath(__file__)))
 seen_files = set()
 out = sys.stdout
 
+# Find a file in the different folders of the src dir.
+def find_file(filename):
+  names = [
+    join(WREN_DIR, 'src', 'include', filename),
+    join(WREN_DIR, 'src', 'vm', filename),
+    join(WREN_DIR, 'src', 'optional', filename),
+  ]
+  for f in names:
+    if isfile(f):
+      return f
+  raise Exception('File "{0}" not found!'.format(filename))
+
 # Prints a plain text file, adding comment markers.
 def add_comment_file(filename):
   with open(filename, 'r') as f:
@@ -26,19 +38,18 @@ def add_file(filename):
   if bname in seen_files:
     return
   once = False
-  path = dirname(filename)
 
-  out.write('// Begin file "{0}"\n'.format(filename))
+  out.write('// Begin file "{0}"\n'.format(bname))
   with open(filename, 'r') as f:
     for line in f:
       m = INCLUDE_PATTERN.match(line)
       if m:
-        add_file(join(path, m.group(1)))
+        add_file(find_file(m.group(1)))
       else:
         out.write(line)
       if GUARD_PATTERN.match(line):
         once = True
-  out.write('// End file "{0}"\n'.format(filename))
+  out.write('// End file "{0}"\n'.format(bname))
 
   # Only skip header files which use #ifndef guards.
   # This is necessary because of the X Macro technique.
@@ -57,3 +68,6 @@ add_file(join(WREN_DIR, 'src', 'vm', 'wren_debug.h'))
 
 for f in iglob(join(WREN_DIR, 'src', 'vm', '*.c')):
   add_file(f)
+
+for f in iglob(join(WREN_DIR, 'src', 'optional', '*.c')):
+  add_file(f)