cmake: always enable warnings without -Werror

Warnings are useful, let's enable them by default.

CMakeLists.txt

@@ -285,12 +285,17 @@ if(HAVE_EPOLL AND HAVE_TIMERFD_CREATE)
   set(ENABLE_TINY_NGHTTPD 1)
 endif()
 
+set(WARNCFLAGS)
+set(WARNCXXFLAGS)
 if(ENABLE_WERROR)
+  extract_valid_c_flags(WARNCFLAGS    -Werror)
+  extract_valid_c_flags(WARNCXXFLAGS  -Werror)
+endif()
+if(NOT CMAKE_C_COMPILER_ID STREQUAL "MSVC")
   # For C compiler
   extract_valid_c_flags(WARNCFLAGS
     -Wall
     -Wextra
-    -Werror
     -Wmissing-prototypes
     -Wstrict-prototypes
     -Wmissing-declarations
@@ -338,11 +343,11 @@ if(ENABLE_WERROR)
   extract_valid_cxx_flags(WARNCXXFLAGS
     # For C++ compiler
     -Wall
-    -Werror
     -Wformat-security
   )
 endif()
 
+set(EXTRACFLAG)
 extract_valid_c_flags(EXTRACFLAG
   -fvisibility=hidden
 )

cmake/ExtractValidFlags.cmake

 # Convenience function that checks the availability
-# of certain C or C++ compiler flags and returns the valid ones.
+# of certain C or C++ compiler flags and appends valid ones.
 
 include(CheckCCompilerFlag)
 include(CheckCXXCompilerFlag)
 
 function(extract_valid_c_flags varname)
-  set(valid_flags "")
   foreach(flag IN LISTS ARGN)
     string(REGEX REPLACE "[^a-zA-Z0-9_]+" "_" flag_var ${flag})
     set(flag_var "C_FLAG_${flag_var}")
@@ -18,7 +17,6 @@ function(extract_valid_c_flags varname)
 endfunction()
 
 function(extract_valid_cxx_flags varname)
-  set(valid_flags "")
   foreach(flag IN LISTS ARGN)
     string(REGEX REPLACE "[^a-zA-Z0-9_]+" "_" flag_var ${flag})
     set(flag_var "CXX_FLAG_${flag_var}")