OpenSSL buildsystem: actually apply the flag on gcc/clang

Commit 972b0f7 was bogus.

drivers/builtin_openssl2/SCsub

@@ -651,9 +651,9 @@ env_ssl.Append(CPPPATH=["#drivers/builtin_openssl2/crypto/modes"])
 #env_ssl.Append(CPPPATH=["#drivers/builtin_openssl2/crypto/store"])
 env_ssl.Append(CPPFLAGS=["-DOPENSSL_NO_ASM","-DOPENSSL_THREADS","-DL_ENDIAN"])
 
-# Workaround for compilation error with MinGW
+# Workaround for compilation error with GCC/Clang when -Werror is too greedy (GH-4517)
 import os
-if (os.name=="nt" and os.getenv("VSINSTALLDIR")==None): # Windows and not MSVC, so likely MinGW
-	env_ssl.Append(CFLAGS=["-Wno-error=implicit-function-declaration"]);
+if not (os.name=="nt" and os.getenv("VSINSTALLDIR")!=None): # not Windows and not MSVC
+	env_ssl.Append(CFLAGS=["-Wno-error=implicit-function-declaration"])
 
 env_ssl.add_source_files(env.drivers_sources,openssl_sources)