contrib: fix wrong toolset used to build boost

This patch fixes regression introduced in commit 6f57f002.
clang++ was used in place of g++ on Android platform.

Change-Id: Idcb7baafa842eb1f68e9a9ed02e5d7949e1588d5
Tuleap: #682

contrib/src/boost/rules.mak

@@ -53,7 +53,7 @@ ifdef HAVE_WIN32
 	cd $< && echo ";" >> user-config.jam
 endif
 ifdef HAVE_ANDROID
-	cd $< && echo "using gcc : android : clang++" > user-config.jam
+	cd $< && echo "using gcc : android : g++" > user-config.jam
 	cd $< && echo ":" >> user-config.jam
 	cd $< && echo "<archiver>${HOST}-ar" >> user-config.jam
 	cd $< && echo "<compileflags>-DANDROID" >> user-config.jam