curl: re-enable UNICODE by default

The article
https://blog.orange.tw/posts/2025-01-worstfit-unveiling-hidden-transformers-in-windows-ansi/
may tip the balance towards UNICODE enabled being the less bad solution.
It mitigates Windows's own shortcomings in translating between Unicode
and 8-bit codepages (aptly coined "WorstFit"). It may also be a way to
receive more feedback on curl UNICODE mode issues, also possibly giving
a new momentum to address them.

Original reasons for disabling, written in
bb02905 still apply today.

Also add a `nounicode` config option to build without UNICODE.

Follow-up to bb02905

Author: vszakats

README.md

@@ -95,7 +95,7 @@ SSL           | ✓       | ✓       | ✓       | ✓       | ✓       | ✓
 SSPI          | ✓       | -       | -       | ✓¹      | ✓¹      | ✓¹      | ✓¹      |         |
 SSLS-EXPORT   | ✓       | ✓       | ✓       | ✓       |         |         |         |         |
 threadsafe    | ✓       | ✓       | ✓       | ✓       | ✓       | ✓       | ✓       | ✓       |
-Unicode       |         | -       | -       |         |         |         |         |         |
+Unicode       | ✓       | -       | -       | ✓¹      | ✓¹      | ✓¹      | ✓¹      | ✓¹      |
 UnixSockets   | ✓       | ✓       | ✓       | ✓       | ✓       | ✓       | ✓       |         |
 zstd          | ✓       | ✓       | ✓       | ✓       |         |         |         |         |
 

_build.sh

@@ -75,7 +75,7 @@ set -o xtrace -o errexit -o nounset; [ -n "${BASH:-}${ZSH_NAME:-}" ] && set -o p
 #        curldocs   include curl Markdown manual pages in the package
 #        msvcrt     build against msvcrt instead of UCRT (for win target)
 #        gcc        build with GCC (including Apple clang aliased to gcc) (defaults to llvm, and "gcc" (= Apple clang) for mac target)
-#        unicode    build curl in UNICODE mode (for win target) [EXPERIMENTAL]
+#        nounicode  build curl without [EXPERIMENTAL] UNICODE mode (for win target)
 #        osnoidn    build curl without OS-supplied IDN support
 #        werror     turn compiler warnings into errors
 #        nodumpconf do not dump curl_config.h

curl.sh

@@ -44,7 +44,7 @@ _VER="$1"
   # [ "${_CC}" = 'gcc' ] && LDFLAGS+=' -Wl,--trace'
   fi
 
-  if [ "${_OS}" = 'win' ] && [[ "${_CONFIG}" = *'unicode'* ]]; then
+  if [ "${_OS}" = 'win' ] && [[ "${_CONFIG}" != *'nounicode'* ]]; then
     options+=' -DENABLE_UNICODE=ON'
   fi