Optional mbedTLS support for embedded / cross-compiled environments

[abin-z]

Hi @yhirose,

First of all, thank you for cpp-httplib.
Its clean design, single-header approach, and minimal dependencies make it a joy to use.

I’m currently working on an embedded / cross-compiled project (e.g. ARM Linux),
where mbedTLS is often the preferred or only available TLS library due to its
smaller footprint and common adoption in embedded toolchains.

At the moment, cpp-httplib enables HTTPS via OpenSSL (and compatible implementations),
which works very well on desktop and server platforms.
Would you consider adding optional mbedTLS support as an alternative TLS backend,
enabled only when explicitly requested?

I completely understand that supporting multiple TLS backends increases complexity
and maintenance cost, and that this may be outside the project’s current scope.
Even a minimal client-side HTTPS integration would already be very valuable
for embedded use cases.

If this is something you’d be open to in principle, I’d be happy to help with testing
or provide feedback from an embedded-user perspective.

Thanks again for your excellent work.

[abin-z]

For example, in a typical Release build:

TLS library	Static library size
OpenSSL (libssl.a + libcrypto.a)	10MB+
mbedTLS	< 2MB

The significant size difference makes mbedTLS much more suitable for memory- and storage-constrained embedded systems.