improve NetBSD-specific code

NetBSD doesn't put packages in /usr/local like /CMakeLists.txt thought.
The `#ifdef __NetBSD__` around iconv was actually breaking compilation
on NetBSD when using the system libiconv (there's also a GNU iconv
package)
A C program included from C++ source broke on NetBSD specifically, work
around it.

This doesn't fix compilation on NetBSD, which is currently broken, but
is closer to correct.

Source/Core/Core/PowerPC/Expression.cpp

@@ -11,6 +11,17 @@
 #include <string_view>
 #include <utility>
 
+// https://github.com/zserge/expr/ is a C program and sorta valid C++.
+// When included in a C++ program, it's treated as a C++ code, and it may cause
+// issues: <cmath> may already be included, if so, including <math.h> may
+// not do anything. <math.h> is obligated to put its functions in the global
+// namespace, while <cmath> may or may not. The C code we're interpreting as
+// C++ won't call functions by their qualified names. The code may work anyway
+// if <cmath> puts its functions in the global namespace, or if the functions
+// are actually macros that expand inline, both of which are common.
+// NetBSD 10.0 i386 is an exception, and we need `using` there.
+using std::isinf;
+using std::isnan;
 #include <expr.h>
 
 #include "Common/BitUtils.h"