Do not build C++ example if a C++ compiler isn't available

By default lttng-ust builds a hello.cxx C++ example that demonstrates
the usage of the userspace tracing library in a C++
program. Unfortunately, when no C++ support is available, the build
of lttng-ust fails just because of this example code. So we make the
compilation of this code conditional on whether a working C++ compiler
was found.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: lttng-ust-1.9.2/configure.ac
===================================================================
--- lttng-ust-1.9.2.orig/configure.ac	2012-02-04 14:49:45.387325814 +0100
+++ lttng-ust-1.9.2/configure.ac	2012-02-04 14:50:08.156920117 +0100
@@ -38,6 +38,18 @@
 AC_PROG_MAKE_SET
 LT_INIT
 
+# rw_PROG_CXX_WORKS
+# Check whether the C++ compiler works.
+AC_CACHE_CHECK([whether the C++ compiler works],
+               [rw_cv_prog_cxx_works],
+               [AC_LANG_PUSH([C++])
+                AC_LINK_IFELSE([AC_LANG_PROGRAM([], [])],
+                               [rw_cv_prog_cxx_works=yes],
+                               [rw_cv_prog_cxx_works=no])
+                AC_LANG_POP([C++])])
+
+AM_CONDITIONAL([CXX_WORKS], [test "x$rw_cv_prog_cxx_works" = "xyes"])
+
 ## Checks for libraries.
 AC_CHECK_LIB([dl], [dlopen])
 AC_CHECK_LIB([pthread], [pthread_create])
Index: lttng-ust-1.9.2/tests/Makefile.am
===================================================================
--- lttng-ust-1.9.2.orig/tests/Makefile.am	2012-02-04 14:49:49.157258640 +0100
+++ lttng-ust-1.9.2/tests/Makefile.am	2012-02-04 14:50:15.096796474 +0100
@@ -1,6 +1,10 @@
-SUBDIRS = . hello fork ust-basic-tracing ust-multi-test demo hello.cxx
+SUBDIRS = . hello fork ust-basic-tracing ust-multi-test demo
 #SUBDIRS = . hello2 basic basic_long simple_include snprintf test-nevents test-libustinstr-malloc dlopen same_line_marker trace_event register_test tracepoint libustctl_function_tests exit-fast
 
+if CXX_WORKS
+SUBDIRS += hello.cxx
+endif
+
 dist_noinst_SCRIPTS = test_loop runtests trace_matches
 
 noinst_LIBRARIES = libtap.a