improve clang attribute detection

Author: odygrd

CHANGELOG.md

@@ -197,6 +197,7 @@
 - On Linux, setting a long backend thread name now truncates it instead of
   failing. ([#691](https://github.com/odygrd/quill/issues/691))
 - Fixed BSD builds. ([#688](https://github.com/odygrd/quill/issues/688))
+- Fixed `QUILL_ATTRIBUTE_HOT` and `QUILL_ATTRIBUTE_COLD` clang detection
 - CMake improvements: switched to range syntax for minimum required version and bumped minimum required CMake version to
   `3.12`. ([#686](https://github.com/odygrd/quill/issues/686))
 - Correct the installation location of pkg-config files. They are now properly placed in `/usr/local/lib`.

include/quill/core/Attributes.h

@@ -67,9 +67,9 @@
  * Portable maybe_unused
  */
 #ifndef QUILL_MAYBE_UNUSED
-  #if QUILL_HAS_CPP_ATTRIBUTE(maybe_unused) && (defined(_HAS_CXX17) && _HAS_CXX17 == 1)
+  #if QUILL_HAS_CPP_ATTRIBUTE(maybe_unused) && (__cplusplus >= 201703L)
     #define QUILL_MAYBE_UNUSED [[maybe_unused]]
-  #elif QUILL_HAS_ATTRIBUTE(__unused__) || defined(__GNUC__)
+  #elif QUILL_HAS_ATTRIBUTE(__unused__)
     #define QUILL_MAYBE_UNUSED __attribute__((__unused__))
   #elif defined(_MSC_VER)
     #define QUILL_MAYBE_UNUSED __pragma(warning(suppress : 4100))
@@ -85,15 +85,15 @@
  * is likely to be not-taken.
  */
 #ifndef QUILL_ATTRIBUTE_HOT
-  #if QUILL_HAS_ATTRIBUTE(hot) || (defined(__GNUC__) && !defined(__clang__))
+  #if QUILL_HAS_ATTRIBUTE(hot)
     #define QUILL_ATTRIBUTE_HOT __attribute__((hot))
   #else
     #define QUILL_ATTRIBUTE_HOT
   #endif
 #endif
 
 #ifndef QUILL_ATTRIBUTE_COLD
-  #if QUILL_HAS_ATTRIBUTE(cold) || (defined(__GNUC__) && !defined(__clang__))
+  #if QUILL_HAS_ATTRIBUTE(cold)
     #define QUILL_ATTRIBUTE_COLD __attribute__((cold))
   #else
     #define QUILL_ATTRIBUTE_COLD
@@ -104,7 +104,7 @@
  * Used
  */
 #ifndef QUILL_ATTRIBUTE_USED
-  #if QUILL_HAS_ATTRIBUTE(used) || defined(__GNUC__) || defined(__clang__)
+  #if QUILL_HAS_ATTRIBUTE(used)
     #define QUILL_ATTRIBUTE_USED __attribute__((used))
   #else
     #define QUILL_ATTRIBUTE_USED