Generate jerryscript-config.h before build (#2934)

Previously after the library was build there was no easy way
to get back the information of how it was build.

This change adds a bit of CMake code to generate the
jerryscript-config.h file containing the default options
and adding the modified build options/features.

JerryScript-DCO-1.0-Signed-off-by: Peter Gal pgal.u-szeged@partner.samsung.com

jerry-core/config.h

@@ -13,8 +13,10 @@
  * limitations under the License.
  */
 
-#ifndef CONFIG_H
-#define CONFIG_H
+#ifndef JERRYSCRIPT_CONFIG_H
+#define JERRYSCRIPT_CONFIG_H
+
+// @JERRY_BUILD_CFG@
 
 /**
  * Built-in configurations
@@ -698,4 +700,4 @@
 #  error "Date does not support float32"
 #endif
 
-#endif /* !CONFIG_H */
+#endif /* !JERRYSCRIPT_CONFIG_H */