trurl: rework how we build trurl [ci skip]

Before this patch we built trurl in `-dev` builds or when explicitly
requrested with `-trurl`. When building curl with `-zero`, trurl linked
libcurl statically, otherwise dynamically. The latter was barely working
with a bunch of half-baked and painful hacks, and the goal of these
builds was always to produce static binaries.

Rework this in this patch to build a second libcurl, specifically
configured to trurl, with a minimal set of option to enable all trurl
functionality, but not more. Then use this minimal libcurl to build
trurl, linking it statically.

The second, minimal libcurl build is quite fast (adding ~15-60 seconds
to jobs), and the statically build trurl is reasonably small (340 KB for
macOS ARM64). Small enough to distribute it alongside curl if we opt
doing so.

The minimal libcurl library itself is not distributed in the binary
package.

The minimal build is build without unity for small binary size.
Build time cost comparison:
Without unity: https://github.com/curl/curl-for-win/actions/runs/18807659190
With unity:    https://github.com/curl/curl-for-win/actions/runs/18808071491

Size difference for macOS arm64 trurl binary:
Without unity: 338248 bytes
With unity:    372488 bytes

The condition to build trurl did not change: needs `-dev` or `-trurl`
option.

Ref: curl/trurl#375

Author: vszakats

_build.sh

@@ -1752,7 +1752,42 @@ build_single_target() {
   bld libssh2           "${LIBSSH2_VER_}"
   bld cacert             "${CACERT_VER_}"
   bld curl                 "${CURL_VER_}"
-  bld trurl               "${TRURL_VER_}"
+
+  # Special build path for trurl. We first build another libcurl tailored
+  # for trurl. This means using no dependencies and just the options necessary
+  # for trurl, to keep binary size low, with static linking.
+  if [[ "${_DEPS}" = *'trurl'* ]]; then
+    save_CONFIG="${_CONFIG}"
+    save_PKGDIR="${_PKGDIR}"
+    save_PKGDIRS="${_PKGDIRS}"
+    save_ZLIB="${_ZLIB}"
+    save_OPENSSL="${_OPENSSL}"
+    save_PP="${_PP}"
+    save_PPS="${_PPS}"
+
+    # Minimal curl configuration optimized for trurl
+    _CONFIG+='-zero-imap-osnotls-osnoidn-nohttp-nocurltool-nounity-CURLNOPKG'
+
+    _PKGDIR="_${_CPU}-${_OS}-${_CRT}-for-trurl"
+    _PKGDIRS="${_PKGDIR}"
+    _ZLIB=
+    _OPENSSL=
+    _PP="${_PKGDIR}${_PREFIX}"
+    _PPS="${_PKGDIRS}${_PREFIX}"
+
+    bld curl                 "${CURL_VER_}"
+    bld trurl               "${TRURL_VER_}"
+
+    _CONFIG="${save_CONFIG}"
+    _PKGDIR="${save_PKGDIR}"
+    _PKGDIRS="${save_PKGDIRS}"
+    _ZLIB="${save_ZLIB}"
+    _OPENSSL="${save_OPENSSL}"
+    _PP="${save_PP}"
+    _PPS="${save_PPS}"
+  else
+    bld trurl               "${TRURL_VER_}"
+  fi
 
   # Unified, per-target package: Build
   export _NAM="${_UNIPKG}"

curl.sh

@@ -408,6 +408,8 @@ _VER="$1"
     TZ=UTC cmake --build "${_BLDDIR}" --target testdeps
   fi
 
+  [[ "${_CONFIG}" = *'CURLNOPKG'* ]] && exit
+
   # Manual copy to DESTDIR
 
   # These custom outputs end up in different directories depending on make tool