Only use noexcept in public headers on >= C++11 (#1071)

This resolves issues consuming openal-soft in projects that don't
explicitly specify a C++ standard of C++11 or newer.

This also handles the fact that __cplusplus is not set properly in MSVC
(always claiming to be 199711L) which prevented both the C++11 noexcept
and the C++17 noexcept annotations from taking effect.

include/AL/al.h

@@ -5,9 +5,19 @@
 #ifdef __cplusplus
 extern "C" {
 
+#ifdef _MSVC_LANG
+#define AL_CPLUSPLUS _MSVC_LANG
+#else
+#define AL_CPLUSPLUS __cplusplus
+#endif
+
 #ifndef AL_DISABLE_NOEXCEPT
+#if AL_CPLUSPLUS >= 201103L
 #define AL_API_NOEXCEPT noexcept
-#if __cplusplus >= 201703L
+#else
+#define AL_API_NOEXCEPT
+#endif
+#if AL_CPLUSPLUS >= 201703L
 #define AL_API_NOEXCEPT17 noexcept
 #else
 #define AL_API_NOEXCEPT17
@@ -19,6 +29,8 @@ extern "C" {
 #define AL_API_NOEXCEPT17
 #endif
 
+#undef AL_CPLUSPLUS
+
 #else /* __cplusplus */
 
 #define AL_API_NOEXCEPT

include/AL/alc.h

@@ -5,9 +5,19 @@
 #ifdef __cplusplus
 extern "C" {
 
+#ifdef _MSVC_LANG
+#define ALC_CPLUSPLUS _MSVC_LANG
+#else
+#define ALC_CPLUSPLUS __cplusplus
+#endif
+
 #ifndef AL_DISABLE_NOEXCEPT
+#if ALC_CPLUSPLUS >= 201103L
 #define ALC_API_NOEXCEPT noexcept
-#if __cplusplus >= 201703L
+#else
+#define ALC_API_NOEXCEPT
+#endif
+#if ALC_CPLUSPLUS >= 201703L
 #define ALC_API_NOEXCEPT17 noexcept
 #else
 #define ALC_API_NOEXCEPT17
@@ -19,6 +29,8 @@ extern "C" {
 #define ALC_API_NOEXCEPT17
 #endif
 
+#undef ALC_CPLUSPLUS
+
 #else /* __cplusplus */
 
 #define ALC_API_NOEXCEPT