Build fixups for Visual Studio 2015 and 2017

Signed-off-by: Nigel Stewart <[email protected]>

src/Imath/ImathPlane.h

@@ -45,7 +45,7 @@ template <class T> class Plane3
     ///	@name Constructors
 
     /// Uninitialized by default
-    IMATH_HOSTDEVICE constexpr Plane3() noexcept {}
+    IMATH_HOSTDEVICE Plane3() noexcept {}
 
     /// Initialize with a normal and distance
     IMATH_HOSTDEVICE IMATH_CONSTEXPR14 Plane3 (const Vec3<T>& normal, T distance) noexcept;

src/Imath/ImathVec.h

@@ -277,7 +277,7 @@ template <class T> class Vec3
     ///	@name Constructors and Assignment
 
     /// Uninitialized by default
-    IMATH_HOSTDEVICE constexpr Vec3() noexcept;
+    IMATH_HOSTDEVICE Vec3() noexcept;
 
     /// Initialize to a scalar `(a,a,a)`
     IMATH_HOSTDEVICE IMATH_CONSTEXPR14 explicit Vec3 (T a) noexcept;
@@ -503,7 +503,7 @@ template <class T> class Vec4
     ///	@name Constructors and Assignment
 
     /// Uninitialized by default
-    IMATH_HOSTDEVICE constexpr Vec4() noexcept;                            // no initialization
+    IMATH_HOSTDEVICE Vec4() noexcept;                            // no initialization
 
     /// Initialize to a scalar `(a,a,a,a)`
     IMATH_HOSTDEVICE IMATH_CONSTEXPR14 explicit Vec4 (T a) noexcept;       // (a a a a)
@@ -1207,7 +1207,7 @@ Vec3<T>::operator[] (int i) const noexcept
     return (&x)[i]; // NOSONAR - suppress SonarCloud bug report.
 }
 
-template <class T> constexpr inline Vec3<T>::Vec3() noexcept
+template <class T> inline Vec3<T>::Vec3() noexcept
 {
     // empty
 }
@@ -1680,7 +1680,7 @@ Vec4<T>::operator[] (int i) const noexcept
     return (&x)[i]; // NOSONAR - suppress SonarCloud bug report.
 }
 
-template <class T> constexpr inline Vec4<T>::Vec4() noexcept
+template <class T> inline Vec4<T>::Vec4() noexcept
 {
     // empty
 }