Enable FPE on Mac (#4213)

Allow #3687 to work on Mac

CMakeLists.txt

@@ -461,7 +461,7 @@ IF(LMMS_BUILD_WIN32)
 ENDIF(LMMS_BUILD_WIN32)
 
 IF(WANT_DEBUG_FPE)
-	IF(LMMS_BUILD_LINUX)
+	IF(LMMS_BUILD_LINUX OR LMMS_BUILD_APPLE)
 		SET(LMMS_DEBUG_FPE TRUE)
 		SET (STATUS_DEBUG_FPE "Enabled")
 	ELSE()

include/fenv.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include_next <fenv.h>
+
+#if defined(__APPLE__) && defined(__MACH__)
+
+// Public domain polyfill for feenableexcept on OS X
+// http://www-personal.umich.edu/~williams/archive/computation/fe-handling-example.c
+
+inline int feenableexcept(unsigned int excepts)
+{
+    static fenv_t fenv;
+    unsigned int new_excepts = excepts & FE_ALL_EXCEPT;
+    // previous masks
+    unsigned int old_excepts;
+
+    if (fegetenv(&fenv)) {
+        return -1;
+    }
+    old_excepts = fenv.__control & FE_ALL_EXCEPT;
+
+    // unmask
+    fenv.__control &= ~new_excepts;
+    fenv.__mxcsr   &= ~(new_excepts << 7);
+
+    return fesetenv(&fenv) ? -1 : old_excepts;
+}
+
+inline int fedisableexcept(unsigned int excepts)
+{
+    static fenv_t fenv;
+    unsigned int new_excepts = excepts & FE_ALL_EXCEPT;
+    // all previous masks
+    unsigned int old_excepts;
+
+    if (fegetenv(&fenv)) {
+        return -1;
+    }
+    old_excepts = fenv.__control & FE_ALL_EXCEPT;
+
+    // mask
+    fenv.__control |= new_excepts;
+    fenv.__mxcsr   |= new_excepts << 7;
+
+    return fesetenv(&fenv) ? -1 : old_excepts;
+}
+
+#endif