The post-patch: section of devel/gobject-introspection makes a supstitution of cc in the file giscanner/sourcescanner.py.

For easy reading I will assume CC="XX XY"

Line 237:
        for filename in filenames:
            if (filename.endswith('.c') or filename.endswith('.cpp') or
                filename.endswith('.cc') or filename.endswith('.cxx')):
                filename = os.path.abspath(filename)
                self._scanner.lex_filename(filename)
            else:
                headers.append(filename)

Turns to:
        for filename in filenames:
            if (filename.endswith('.c') or filename.endswith('.cpp') or
                filename.endswith('.XX YY') or filename.endswith('.cxx')):
                filename = os.path.abspath(filename)
                self._scanner.lex_filename(filename)
            else:
                headers.append(filename)

I think it's clear why that isn't a good idea.

The next affected line is 273:
        cpp_args = ['cc', '-E', '-C', '-I.', '-']

        cpp_args += self._cpp_options
        proc = subprocess.Popen(cpp_args,
                                stdin=subprocess.PIPE,
                                stdout=subprocess.PIPE)

Turns to:
        cpp_args = ['XX YY', '-E', '-C', '-I.', '-']

        cpp_args += self._cpp_options
        proc = subprocess.Popen(cpp_args,
                                stdin=subprocess.PIPE,
                                stdout=subprocess.PIPE)

However the subprocess.Popen constructor requires the following form:
        cpp_args = ['XX', 'YY', '-E', '-C', '-I.', '-']

Fix: The attached patch causes line 239 no longer to be touched and performs the correct substitution for subprocess.Popen in line 273.

Patch attached with submission follows:
How-To-Repeat: Build with no-default CC, to trigger a build fail use CC with spaces.