Tolerate TS 18661 (_Float32 et al) types

Because we currently claim GCC 4.2.1 support, but also define the TS 18661 floating point types,
we confuse Glibc 2.27+, which checks for GCC >= 7.0 and otherwise defines the types itself.
In the long run we should either upgrade our support to GCC 7.0 standard, or else stop providing
these types and simply ask users interested in these types to use Glibc 2.27 or provide their own
headers; in the meantime this workaround permits us to avoid crashing whenever math.h is included.

Author: smowton

src/ansi-c/ansi_c_convert_type.cpp

@@ -344,9 +344,17 @@ void ansi_c_convert_typet::write(typet &type)
        gcc_float64_cnt+gcc_float64x_cnt+
        gcc_float128_cnt+gcc_float128x_cnt>=2)
     {
-      error().source_location=source_location;
-      error() << "conflicting type modifiers" << eom;
-      throw 0;
+      if(c_storage_spec.is_typedef)
+      {
+        warning().source_location = source_location;
+        warning() << "ignoring typedef for TS 18661 (_FloatNNx) type" << eom;
+      }
+      else
+      {
+        error().source_location=source_location;
+        error() << "conflicting type modifiers" << eom;
+        throw 0;
+      }
     }
 
     // _not_ the same as float, double, long double