Suppress compiler warnings with OpenSSL < 3.0 and fix engine-less builds

[olszomal]

This PR adds conditional diagnostic pragmas to locally suppress -Wcast-qual and -Wdiscarded-qualifiers warnings triggered by legacy OpenSSL APIs (BIO_set_md, PKCS7_dup, i2d_X509) which discard const qualifiers. These warnings are harmless, as OpenSSL does not modify the data. Suppressing them locally avoids build noise while preserving correctness and portability.

Additionally, this fixes build failures with OpenSSL ≥ 3.0 when engines are disabled via OPENSSL_NO_ENGINE or OPENSSL_NO_DEPRECATED.
Fixes #430.

[mtrojnar]

clang also defines __GNUC__ and supports #pragma GCC, so there is no need for separate #if defined(__clang__) cases.

[olszomal]

You're right that Clang defines __GNUC__ and supports some #pragma GCC directives. However, in this case Clang does not recognize -Wdiscarded-qualifiers and emits the following warning:

/home/mo/osslsigncode/helpers.c:783:32: warning: unknown warning group '-Wdiscarded-qualifiers', ignored [-Wunknown-warning-option]
#pragma GCC diagnostic ignored "-Wdiscarded-qualifiers"
                               ^

Because of this, separate handling for Clang is necessary. Clang requires the use of -Wincompatible-pointer-types-discards-qualifiers instead. Splitting the conditions ensures the correct suppression is applied without introducing new build warnings on either compiler.