Add code to determine a version string

If the folder where the code is compiled is a valid git repository, the
version string is generated with `git describe`. If it isn't a valid
repository, a string included in the source code is used instead. This
one must be updated regularly as the toolchain is developed.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

include/extern/version.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2017 Antonio Nino Diaz <antonio_nd@outlook.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#define PACKAGE_VERSION_MAJOR (0)
+#define PACKAGE_VERSION_MINOR (3)
+#define PACKAGE_VERSION_PATCH (2)
+
+const char * get_package_version_string(void);