iverilog-vpi.sh: Put IVCXX in quotes to allow to pass arguments

`IVCXX` which contains the C++ compiler that will be invoked when building
an vpi module might contain additional arguments that get passed to the
compiler. E.g. such as the C++ version (`-std=c++11`). For this to work
properly `IVCXX` needs to be put in quotes.

This fixes intermittent CI failures for the MacOS target.

Signed-off-by: Lars-Peter Clausen <[email protected]>

Author: larsclausen

iverilog-vpi.sh

@@ -20,7 +20,7 @@
 
 # These are the variables used for compiling files
 CC="@IVCC@"
-CXX=@IVCXX@
+CXX="@IVCXX@"
 CFLAGS="@PIC@ @IVCFLAGS@ -I@INCLUDEDIR@"
 CXXFLAGS="@PIC@ @IVCXXFLAGS@ -I@INCLUDEDIR@"