[core] preprocessor option to force crypto lib
[core] preprocessor option to force crypto lib

add preprocessor option to force crypto lib used by lighttpd base
  -DFORCE_MBEDTLS_CRYPTO for mbed TLS
  or -DFORCE_WOLFSSL_CRYPTO for WolfSSL

This allows building lighttpd with support for multiple TLS modules,
and at the same time specifying the lighttpd base crypto lib dependency.

(Packages such as OpenWRT lighttpd package would need to update
package dependency along with specifying the preprocessor define)

If lighttpd is built with meson and without "-Dwith_nettle=true",
then lighttpd will use mbed TLS if built with "-Dwith_mbedtls",
so the preprocessor option is not strictly necessary for mbed TLS.
However, if built with "-Dwith_mbedtls" and "-Dwith_wolfssl" *and*
without "-Dwith_nettle=true" *and* lighttpd base crypto dependency
on WolfSSL is desired, then CFLAGS -DFORCE_WOLFSSL_CRYPTO is needed,
e.g. with a patch to add it to 'defs' near the top of src/meson.build.

x-ref:
  "lighttpd authentication modules depends on libnettle"
  https://github.com/openwrt/packages/issues/18851

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4