tests: fix 'make check' build for setups where no c++ compiler is available

Only try to build (pseudo-)C++ unit test if a working C++ compiler has been
found, otherwise the build will fail. (We do this to make sure our headers
are 'C++ clean').

configure.ac

@@ -228,10 +228,15 @@ AM_PROG_CC_C_O
 dnl find an assembler
 AM_PROG_AS
 
-dnl determine c++ compiler
-AC_PROG_CXX
 dnl determine if c++ is available on this system
-AC_CHECK_PROG(HAVE_CXX, $CXX, yes, no)
+AC_PROG_CXX
+dnl CXX may be set to some default even if no c++ compiler is available
+dnl (thanks autotools!), so just try to compile some c++ code to make sure
+AC_LANG_PUSH([C++])
+AC_TRY_COMPILE([ class Foo { int bar; };], , working_cxx=yes, working_cxx=no)
+AC_LANG_POP([C++])
+AC_MSG_NOTICE([working c++ compiler found: $working_cxx])
+AM_CONDITIONAL(HAVE_CXX, test "x$working_cxx" == "xyes")
 
 dnl Perl is used in building documentation and in the version checks
 AC_PATH_PROG(PERL_PATH, perl, no)

tests/check/Makefile.am

@@ -43,6 +43,11 @@ endif
 endif
 endif
 
+if HAVE_CXX
+CXX_CHECKS = gst/gstcpp
+else
+CXX_CHECKS =
+endif
 
 # if it's calling gst_element_factory_make(), it will probably not work without
 # a registry
@@ -89,7 +94,7 @@ check_PROGRAMS =				\
 	gst/gstbufferlist			\
 	gst/gstbus				\
 	gst/gstcaps     			\
-	gst/gstcpp			     	\
+	$(CXX_CHECKS)			     	\
 	gst/gstdatetime     			\
 	gst/gstinfo		 		\
 	gst/gstiterator 			\