The 5.4.0 update changed the default bignum implementation to what
could explicitly be enabled via `--enable-sp-math-all`. Since this uses
fixed-sized buffers sufficient for key sizes of SP_INT_BITS, with a default
of 4096, modp6144 and modp8192 didn't work anymore (wc_DhGenerateKeyPair()
returned MP_EXPTMOD_E). So we have to adapt the feature checks for this.
To support the larger DH groups we can either increase the buffer size
via `--with-max-rsa-bits` or add `--enable-heapmath` so buffers get
(re-)allocated as needed. We go with the latter for now.
Use the same configure options etc. for both builds (no need for the cert
options as we don't use TLS or X.509 parsing) and switch to a Git commit
that includes the SHA-3 OID fix (it's actually the fix itself).
