configure --enable-debugging works with C++ sources as well

configure.ac

@@ -136,16 +136,24 @@ AC_ARG_ENABLE(debugging,
 	[],
 	[enable_debugging=no])
 
-# AC_PROG_CC sets CFLAGS to '-g -O2' unless it is already set, so
-# there's no need to add -g. Disable or change by explicitly setting
-# CFLAGS. If this option is enabled, then -Og or -O0 becomes the last
-# optimization option, and hence takes precedence.
+# AC_PROG_CC and AC_PROG_CXX set CFLAGS and CXXFLAGS to '-g -O2'
+# unless already set, so there's no need to add -g. Disable or change
+# by explicitly setting CFLAGS and/or CXXFLAGS. If this option is
+# enabled, then -Og or -O0 becomes the last optimization option, and
+# hence takes precedence.
 if test "x$enable_debugging" != "xno"; then
 	CFLAGS="${CFLAGS} -fno-inline"
 	AX_CHECK_COMPILE_FLAG([-Og],
 		[CFLAGS="${CFLAGS} -Og"],
 		[CFLAGS="${CFLAGS} -O0"],
 		[])
+	AC_LANG_PUSH(C++)
+	CXXFLAGS="${CXXFLAGS} -fno-inline"
+	AX_CHECK_COMPILE_FLAG([-Og],
+		[CXXFLAGS="${CXXFLAGS} -Og"],
+		[CXXFLAGS="${CXXFLAGS} -O0"],
+		[])
+	AC_LANG_POP()
 fi
 
 AC_CONFIG_FILES([