Travis: use enable-buildtest-c++

Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/8370)

(cherry picked from commit 26a053d195d5cc8a5cd648da3f05d3ff0e47f776)

.travis.yml

@@ -28,7 +28,7 @@ compiler:
 env:
     - CONFIG_OPTS="" DESTDIR="_install"
     - CONFIG_OPTS="no-asm -Werror --debug no-afalgeng no-shared enable-crypto-mdebug enable-rc5 enable-md2"
-    - CONFIG_OPTS="no-asm no-makedepend --strict-warnings -D_DEFAULT_SOURCE" BUILDONLY="yes" CHECKDOCS="yes" GENERATE="yes" CPPFLAGS="-ansi"
+    - CONFIG_OPTS="no-asm no-makedepend enable-buildtest-c++ --strict-warnings -D_DEFAULT_SOURCE" BUILDONLY="yes" CHECKDOCS="yes" GENERATE="yes" CPPFLAGS="-ansi"
 
 matrix:
     include: