build.rs: Set compiler C and CPP (preprocessor) flags in one place.

Apparently it is OK to set `-std=c1x` even when compiling assembly
code, so just set it no matter what we're compiling. This simplifies
the code and allows future simplification.

It's not clear why certain warnings were separated from the others.
Combine them too, for the same reasons.

build.rs

@@ -102,27 +102,15 @@ const RING_TEST_SRCS: &[&str] = &[("crypto/constant_time_test.c")];
 
 const PREGENERATED: &str = "pregenerated";
 
-fn c_flags(compiler: &cc::Tool) -> &'static [&'static str] {
-    if !compiler.is_like_msvc() {
-        static NON_MSVC_FLAGS: &[&str] = &[
-            "-std=c1x", // GCC 4.6 requires "c1x" instead of "c11"
-            "-Wbad-function-cast",
-            "-Wnested-externs",
-            "-Wstrict-prototypes",
-        ];
-        NON_MSVC_FLAGS
-    } else {
-        &[]
-    }
-}
-
 fn cpp_flags(compiler: &cc::Tool) -> &'static [&'static str] {
     if !compiler.is_like_msvc() {
         static NON_MSVC_FLAGS: &[&str] = &[
+            "-std=c1x", // GCC 4.6 requires "c1x" instead of "c11"
             "-pedantic",
             "-pedantic-errors",
             "-Wall",
             "-Wextra",
+            "-Wbad-function-cast",
             "-Wcast-align",
             "-Wcast-qual",
             "-Wconversion",
@@ -133,10 +121,12 @@ fn cpp_flags(compiler: &cc::Tool) -> &'static [&'static str] {
             "-Winvalid-pch",
             "-Wmissing-field-initializers",
             "-Wmissing-include-dirs",
+            "-Wnested-externs",
             "-Wredundant-decls",
             "-Wshadow",
             "-Wsign-compare",
             "-Wsign-conversion",
+            "-Wstrict-prototypes",
             "-Wundef",
             "-Wuninitialized",
             "-Wwrite-strings",
@@ -584,12 +574,7 @@ fn cc(file: &Path, ext: &str, target: &Target, include_dir: &Path, out_file: &Pa
     let _ = c.include("include");
     let _ = c.include(include_dir);
     match ext {
-        "c" => {
-            for f in c_flags(&compiler) {
-                let _ = c.flag(f);
-            }
-        }
-        "S" => (),
+        "c" | "S" => (),
         e => panic!("Unsupported file extension: {:?}", e),
     };
     for f in cpp_flags(&compiler) {