Bug 1559301 - Straighten up C/C++ standard compiler flags. r=nalexander

- `info.type in ('clang-cl', 'clang', 'gcc')` is always true since MSVC
support was removed.
- For some reason, we didn't enforce C++14 on GCC.

Differential Revision: https://phabricator.services.mozilla.com/D34990

--HG--
extra : moz-landing-system : lando

build/moz.configure/toolchain.configure

@@ -521,22 +521,22 @@ def check_compiler(compiler, language, target):
     # C99 compiles as e.g. C11 (as of writing, this is true of libnestegg, for
     # example)
     if info.language == 'C' and info.language_version != 199901:
-        if info.type in ('clang-cl', 'clang', 'gcc'):
-            if info.type == 'clang-cl':
-                flags.append('-Xclang')
-            flags.append('-std=gnu99')
+        if info.type == 'clang-cl':
+            flags.append('-Xclang')
+        flags.append('-std=gnu99')
 
     # Note: this is a strict version check because we used to always add
     # -std=gnu++14.
     cxx14_version = 201402
     if info.language == 'C++':
-        if info.type == 'clang' and info.language_version != cxx14_version:
-            flags.append('-std=gnu++14')
-        # MSVC headers include C++14 features, but don't guard them
-        # with appropriate checks.
-        elif info.type == 'clang-cl' and info.language_version != cxx14_version:
-            flags.append('-Xclang')
-            flags.append('-std=c++14')
+        if info.language_version != cxx14_version:
+            # MSVC headers include C++14 features, but don't guard them
+            # with appropriate checks.
+            if info.type == 'clang-cl':
+                flags.append('-Xclang')
+                flags.append('-std=c++14')
+            else:
+                flags.append('-std=gnu++14')
 
     # Check compiler target
     # --------------------------------------------------------------------