Merge bitcoin-core/secp256k1#1084: ci: Add MSVC builds

49e2acd927 configure: Improve rationale for WERROR_CFLAGS (Tim Ruffing)
8dc4b03341 ci: Add a C++ job that compiles the public headers without -fpermissive (Tim Ruffing)
51f296a46c ci: Run persistent wineserver to speed up wine (Tim Ruffing)
3fb3269c22 ci: Add 32-bit MinGW64 build (Tim Ruffing)
9efc2e5221 ci: Add MSVC builds (Tim Ruffing)
2be6ba0fed configure: Convince autotools to work with MSVC's archiver lib.exe (Tim Ruffing)
bd81f4140a schnorrsig bench: Suppress a stupid warning in MSVC (Tim Ruffing)
09f3d71c51 configure: Add a few CFLAGS for MSVC (Tim Ruffing)
3b4f3d0d46 build: Reject C++ compilers in the preprocessor (Tim Ruffing)
1cc0941414 configure: Don't abort if the compiler does not define __STDC__ (Tim Ruffing)
cca8cbbac8 configure: Output message when checking for valgrind (Tim Ruffing)
1a6be5745f bench: Make benchmarks compile on MSVC (Tim Ruffing)

Pull request description:

ACKs for top commit:
  jonasnick:
    ACK 49e2acd927

Tree-SHA512: 986c498fb218231fff3519167d34a92e11dea6a4383788a9723be105c20578cd483c6b06ba5686c6669e3a02cfeebc29b8e5f1428552ebf4ec67fa7a86957548

.cirrus.yml

@@ -241,17 +241,57 @@ task:
   << : *CAT_LOGS
 
 task:
-  name: "x86_64 (mingw32-w64): Windows (Debian stable, Wine)"
   << : *LINUX_CONTAINER
   env:
-    WRAPPER_CMD: wine64-stable
-    SECP256K1_TEST_ITERS: 16
-    HOST: x86_64-w64-mingw32
+    WRAPPER_CMD: wine
     WITH_VALGRIND: no
     ECDH: yes
     RECOVERY: yes
     SCHNORRSIG: yes
     CTIMETEST: no
+  matrix:
+    - name: "x86_64 (mingw32-w64): Windows (Debian stable, Wine)"
+      env:
+        HOST: x86_64-w64-mingw32
+    - name: "i686 (mingw32-w64): Windows (Debian stable, Wine)"
+      env:
+        HOST: i686-w64-mingw32
+  << : *MERGE_BASE
+  test_script:
+    - ./ci/cirrus.sh
+  << : *CAT_LOGS
+
+task:
+  << : *LINUX_CONTAINER
+  env:
+    WRAPPER_CMD: wine
+    WERROR_CFLAGS: -WX
+    WITH_VALGRIND: no
+    ECDH: yes
+    RECOVERY: yes
+    EXPERIMENTAL: yes
+    SCHNORRSIG: yes
+    CTIMETEST: no
+    # Set non-essential options that affect the CLI messages here.
+    # (They depend on the user's taste, so we don't want to set them automatically in configure.ac.)
+    CFLAGS: -nologo -diagnostics:caret
+    LDFLAGS: -XCClinker -nologo -XCClinker -diagnostics:caret
+  # Use a MinGW-w64 host to tell ./configure we're building for Windows.
+  # This will detect some MinGW-w64 tools but then make will need only
+  # the MSVC tools CC, AR and NM as specified below.
+  matrix:
+    - name: "x86_64 (MSVC): Windows (Debian stable, Wine)"
+      env:
+        HOST: x86_64-w64-mingw32
+        CC: /opt/msvc/bin/x64/cl
+        AR: /opt/msvc/bin/x64/lib
+        NM: /opt/msvc/bin/x64/dumpbin -symbols -headers
+    - name: "i686 (MSVC): Windows (Debian stable, Wine)"
+      env:
+        HOST: i686-w64-mingw32
+        CC: /opt/msvc/bin/x86/cl
+        AR: /opt/msvc/bin/x86/lib
+        NM: /opt/msvc/bin/x86/dumpbin -symbols -headers
   << : *MERGE_BASE
   test_script:
     - ./ci/cirrus.sh
@@ -302,13 +342,12 @@ task:
   << : *CAT_LOGS
 
 task:
-  name: "C++ -fpermissive"
+  name: "C++ -fpermissive (entire project)"
   << : *LINUX_CONTAINER
   env:
-    # ./configure correctly errors out when given CC=g++.
-    # We hack around this by passing CC=g++ only to make.
-    CC: gcc
-    MAKEFLAGS: -j4 CC=g++ CFLAGS=-fpermissive\ -g
+    CC: g++
+    CFLAGS: -fpermissive -g
+    CPPFLAGS: -DSECP256K1_CPLUSPLUS_TEST_OVERRIDE
     WERROR_CFLAGS:
     ECDH: yes
     RECOVERY: yes
@@ -318,6 +357,14 @@ task:
     - ./ci/cirrus.sh
   << : *CAT_LOGS
 
+task:
+  name: "C++ (public headers)"
+  << : *LINUX_CONTAINER
+  test_script:
+    - g++ -Werror include/*.h
+    - clang -Werror -x c++-header include/*.h
+    - /opt/msvc/bin/x64/cl.exe -c -WX -TP include/*.h
+
 task:
   name: "sage prover"
   << : *LINUX_CONTAINER