android/build.py: always build with clang

Support for GCC has been dropped from NDK r13.  Let's switch
everything to clang before they really remove GCC completely.

python/build/project.py

@@ -7,7 +7,7 @@ from build.tar import untar
 class Project:
     def __init__(self, url, md5, installed, name=None, version=None,
                  base=None,
-                 use_cxx=False, use_clang=False):
+                 use_cxx=False):
         if base is None:
             basename = os.path.basename(url)
             m = re.match(r'^(.+)\.(tar(\.(gz|bz2|xz|lzma))?|zip)$', basename)
@@ -29,7 +29,6 @@ class Project:
         self.installed = installed
 
         self.use_cxx = use_cxx
-        self.use_clang = use_clang
 
     def download(self, toolchain):
         return download_and_verify(self.url, self.md5, toolchain.tarball_path)