Build option

[fpistm]

It is now possible to customize some core definitions
or gcc options thanks a file named "build_opt.h"
in the sketch directory. (using @file gcc option)

User can add it thanks the Arduino IDE in a tab to easily customize the core options.
That's why '.h' extension is used as IDE allows only those one: ino, h, hpp, c, cpp, pde.
Note: file is automatically copied in the {build.path}/sketch/ directory

Example of file content to change F_CPU value:
-UF_CPU -DF_CPU=72000000UL

If the file does not exist an empty one is created in the {build.path}/sketch/ directory.

Fix #41

[fpistm]

One example using the STM32Ethernet and LwIP libraries:
Adding in build_opt.h:
-DLWIP_DEBUG=1 -DLWIP_DBG_TYPES_ON=LWIP_DBG_ON -DIP_DEBUG=LWIP_DBG_ON "-DAPI_LIB_DEBUG LWIP_DBG_ON"
Allow to enable some debug without need to edit core or libraries files. Only at sketch level.

But there is a limitation. Even if the "Aggressively cache compiled core" is disabled.
When project has been built one time, changing build option in the file will not be taken into account as build see no change on source file (Using previously compiled file)
Only way to take it into account is to reopen IDE or clean the tmp build dir.