examples: C/C++ examples use transitive dependencies

Updated the examples to comprehend transitive dependencies.  This means
that each example target will no longer have a giant list of -I includes
(the examples at the end of the list had includes for all previous
examples, upwards of 200 -I's on the command line).

    * Created a CMakeLists.txt in the upm/examples directory, moved
      common functionality to this level.
    * C/C++ examples now look to the filename for their dependency
      target name, ie; gas-mq2.cxx adds a dependency to the 'gas' target
    * Updated a handful of C/C++ example names to reflect this
    * Example CMake flow - glob the list of files, add targets for any
      special case examples, then att targets for all the rest

Signed-off-by: Noel Eck <noel.eck@intel.com>

examples/c/button-intr.c

@@ -0,0 +1,54 @@
+/*
+ * Authors: Abhishek Malik <abhishek.malik@intel.com>
+ * Copyright (c) 2014 - 2016 Intel Corporation.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining
+ * a copy of this software and associated documentation files (the
+ * "Software"), to deal in the Software without restriction, including
+ * without limitation the rights to use, copy, modify, merge, publish,
+ * distribute, sublicense, and/or sell copies of the Software, and to
+ * permit persons to whom the Software is furnished to do so, subject to
+ * the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
+ * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
+ * LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
+ * OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include "button.h"
+
+#include "upm_utilities.h"
+#include "mraa.h"
+
+int main()
+{
+    if (mraa_init() != MRAA_SUCCESS)
+    {
+        perror("Failed to initialize mraa\n");
+        return -1;
+    }
+
+    button_context dev = button_init(2);
+    bool abc = 0;
+    while(1){
+        if(button_get_value(dev, &abc) != UPM_SUCCESS){
+            printf("an error has occurred\n");
+        }
+        upm_delay(1);
+        printf("value retrieved: %d\n", abc);
+    }
+
+    return 0;
+}