Since the core version 1.1.1, it is possible to customize some core definitions or compiler options by adding a file named "build_opt.h" in the sketch directory.

This file allow to used the "@file" parameter of gcc.

See https://gcc.gnu.org/onlinedocs/gcc/Overall-Options.html

@file

Read command-line options from file. The options read are inserted in place of the original @file option.
If file does not exist, or cannot be read, then the option will be treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option
by surrounding the entire option in either single or double quotes. Any character (including a backslash)
may be included by prefixing the character to be included with a backslash. The file may itself contain
additional @file options; any such options will be processed recursively.

By default, if the file does not exist an empty one is created using the pre-build hooks feature.

• Enable an HAL module:

  -DHAL_UART_MODULE_ENABLED

• Disable an HAL module:

  -UHAL_UART_MODULE_ENABLED

• Enable an Optimization Options:

  -faggressive-loop-optimizations

• Change one default define as the Serial Rx/Tx buffer size:

  -DSERIAL_RX_BUFFER_SIZE=256 -DSERIAL_TX_BUFFER_SIZE=256

If you made a change in the "build_opt.h" file after a first build, this will not be detected by the Arduino IDE and will use the previous build object or cache core to build. To ensure the change is properly used, close the Arduino IDE or change one option in the menu (example upload method). This will force to rebuild all. If the compilation verbose is enabled you should see this at the beginning of the build:

Build options changed, rebuilding all

Else you will see several:

Using previously compiled file: