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

build-aux/m4/bitcoin_secp.m4

@@ -10,6 +10,7 @@ AC_MSG_RESULT([$has_64bit_asm])
 ])
 
 AC_DEFUN([SECP_VALGRIND_CHECK],[
+AC_MSG_CHECKING([for valgrind support])
 if test x"$has_valgrind" != x"yes"; then
   CPPFLAGS_TEMP="$CPPFLAGS"
   CPPFLAGS="$VALGRIND_CPPFLAGS $CPPFLAGS"
@@ -21,6 +22,7 @@ if test x"$has_valgrind" != x"yes"; then
     #endif
   ]])], [has_valgrind=yes; AC_DEFINE(HAVE_VALGRIND,1,[Define this symbol if valgrind is installed, and it supports the host platform])])
 fi
+AC_MSG_RESULT($has_valgrind)
 ])
 
 dnl SECP_TRY_APPEND_CFLAGS(flags, VAR)