Default to stricter warning flags in CMakeLists.txt

[sezrab]

As suggested in #9317, added -std=c99 -pedantic to CMAKE_C_FLAGS in CmakeLists.txt. This has introduced some pointer conversion errors, which are specified below.

metatest.c

/var/lib/build/programs/test/metatest.c:131:34: error: format specifies type 'void *' but the argument has type 'volatile char *' [-Werror,-Wformat-pedantic]
    mbedtls_printf("%p -> %u\n", p, (unsigned) *p);

dlopen.c

/var/lib/build/programs/test/dlopen.c:50:18: error: initializing 'const int *(*)(void)' with an expression of type 'void *' converts between void pointer and function pointer [-Werror,-Wpedantic]
    const int *(*ssl_list_ciphersuites)(void) =
                   
/var/lib/build/programs/test/dlopen.c:78:18: error: initializing 'const int *(*)(void)' with an expression of type 'void *' converts between void pointer and function pointer [-Werror,-Wpedantic]
    const int *(*md_list)(void) =

test_suite_platform.function

/var/lib/build/tests/suites/test_suite_platform.function:130:45: error: format specifies type 'void *' but the argument has type 'unsigned char *' [-Werror,-Wformat-pedantic]
    mbedtls_printf("calloc pointer : %p\n", buf);

PR checklist

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• changelog not required
• development PR provided
• framework PR not required
• 3.6 PR todo
• 2.28 PR not required?
• tests not required