core/assert: provide static_assert for c99

[ZetaR60]

This provides a working static_assert for c versions < c11. Should work globally and locally.

Required by #9190 for detecting collisions in gpio_t definitions.

Example return:

[liveuser@localhost-live driver_gpio_exp_coll]$ make BOARD=mega-xplained
Building application "tests_driver_gpio_exp_coll" for "mega-xplained" with MCU "atmega1284p".

In file included from /home/liveuser/Desktop/RIOT_gpio_exp/tests/driver_gpio_exp_coll/main.c:20:0:
/home/liveuser/Desktop/RIOT_gpio_exp/core/include/assert.h:123:48: error: division by zero [-Werror=div-by-zero]
     enum { static_assert_failed_on_div_by_0 = 1/(!!(cond)) }
                                               ~^~~~~~~~~~~
/home/liveuser/Desktop/RIOT_gpio_exp/tests/driver_gpio_exp_coll/main.c:202:1: note: in expansion of macro 'static_assert'
 static_assert(COLL_RES, "[driver_gpio_exp_coll] Collision in GPIO_EXP_PIN");
 ^~~~~~~~~~~~~
/home/liveuser/Desktop/RIOT_gpio_exp/core/include/assert.h:123:12: error: enumerator value for 'static_assert_failed_on_div_by_0' is not an integer constant
     enum { static_assert_failed_on_div_by_0 = 1/(!!(cond)) }
            ^
/home/liveuser/Desktop/RIOT_gpio_exp/tests/driver_gpio_exp_coll/main.c:202:1: note: in expansion of macro 'static_assert'
 static_assert(COLL_RES, "[driver_gpio_exp_coll] Collision in GPIO_EXP_PIN");
 ^~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[1]: *** [/home/liveuser/Desktop/RIOT_gpio_exp/Makefile.base:83: /home/liveuser/Desktop/RIOT_gpio_exp/tests/driver_gpio_exp_coll/bin/mega-xplained/application_tests_driver_gpio_exp_coll/main.o] Error 1
make: *** [/home/liveuser/Desktop/RIOT_gpio_exp/tests/driver_gpio_exp_coll/../../Makefile.include:359: /home/liveuser/Desktop/RIOT_gpio_exp/tests/driver_gpio_exp_coll/bin/mega-xplained/application_tests_driver_gpio_exp_coll.a] Error 2

[jnohlgard]

Clever solution, I like it! Minor code style comments

[jnohlgard]

Remove the semicolon. The user will write a semicolon when using it

[jnohlgard]

Put spaces around the division operator

[jnohlgard]

ACK, please squash

[ZetaR60]

Thanks, but I can't take credit. I looked for a way of doing it via a web search and this was suggested in several places as a way of implementing static_assert. The more common way of doing it apparently is by a typedef for an array with a negative number of members if the condition fails, but because RIOT is set up to be very strict about code quality it gives a warning about an unused typedef. The enum doesn't give any warnings when it is unused.

[ZetaR60]

@gebart Squashed and did a double-check to make sure it still works correctly with the semicolon removed. Should be good to go.

[kYc0o]

Everything is good here. Let's go!