Remove a bunch og gcc-8 warnings

simgear/math/SGMatrix.hxx

@@ -142,10 +142,10 @@ public:
   T (&sg(void))[4][4]
   { return _data.ptr(); }
   /// Readonly raw storage interface
-  const simd4x4_t<T,4> (&simd4x4(void) const)
+  const simd4x4_t<T,4> &simd4x4(void) const
   { return _data; }
   /// Readonly raw storage interface
-  simd4x4_t<T,4> (&simd4x4(void))
+  simd4x4_t<T,4> &simd4x4(void)
   { return _data; }
 
 

simgear/math/SGVec2.hxx

@@ -87,10 +87,10 @@ public:
   /// Access raw data
   T (&data(void))[2]
   { return _data.ptr(); }
-  const simd4_t<T,2> (&simd2(void) const)
+  const simd4_t<T,2> &simd2(void) const
   { return _data; }
   /// Readonly raw storage interface
-  simd4_t<T,2> (&simd2(void))
+  simd4_t<T,2> &simd2(void)
   { return _data; }
 
   /// Inplace addition

simgear/math/SGVec3.hxx

@@ -106,10 +106,10 @@ public:
   T (&data(void))[3]
   { return _data.ptr(); }
   /// Readonly raw storage interface
-  const simd4_t<T,3> (&simd3(void) const)
+  const simd4_t<T,3> &simd3(void) const
   { return _data; }
   /// Readonly raw storage interface
-  simd4_t<T,3> (&simd3(void))
+  simd4_t<T,3> &simd3(void)
   { return _data; }
 
   /// Inplace addition

simgear/math/SGVec4.hxx

@@ -99,10 +99,10 @@ public:
   T (&data(void))[4]
   { return _data.ptr(); }
   /// Readonly raw storage interface
-  const simd4_t<T,4> (&simd4(void) const)
+  const simd4_t<T,4> &simd4(void) const
   { return _data; }
   /// Readonly raw storage interface
-  simd4_t<T,4> (&simd4(void))
+  simd4_t<T,4> &simd4(void)
   { return _data; }
 
   /// Inplace addition

simgear/math/simd.hxx

@@ -352,10 +352,10 @@ public:
     simd4_t(const simd4_t<float,2>& v) { simd4 = v.v4(); }
     simd4_t(const __m128& v) { simd4 = v; }
 
-    inline const __m128 (&v4(void) const) {
+    inline const __m128 &v4(void) const {
         return simd4;
     }
-    inline __m128 (&v4(void)) {
+    inline __m128 &v4(void) {
         return simd4;
     }
 
@@ -1120,11 +1120,11 @@ public:
     simd4_t(const simd4_t<int,2>& v) { simd4 = v.v4(); }
     simd4_t(const __m128i& v) { simd4 = v; }
 
-    inline __m128i (&v4(void)) {
+    inline __m128i &v4(void) {
         return simd4;
     }
 
-    inline const __m128i (&v4(void) const) {
+    inline const __m128i &v4(void) const {
         return simd4;
     }