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.

CMakeLists.txt

@@ -402,7 +402,7 @@ endif()
 # All commands and submodule commands also need to see these
 # changes, so just setting them in the project scope via
 # include_directories and link_directories is not sufficient
-if(CMAKE_SYSTEM_NAME MATCHES "FreeBSD|NetBSD")
+if(CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
   set(CMAKE_PREFIX_PATH "${CMAKE_PREFIX_PATH};/usr/local")
   set(CMAKE_REQUIRED_INCLUDES "/usr/local/include")
   set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -L/usr/local/lib")

Source/Core/Common/StringUtil.cpp

@@ -35,9 +35,6 @@
 constexpr u32 CODEPAGE_SHIFT_JIS = 932;
 constexpr u32 CODEPAGE_WINDOWS_1252 = 1252;
 #else
-#if defined(__NetBSD__)
-#define LIBICONV_PLUG
-#endif
 #include <errno.h>
 #include <iconv.h>
 #include <locale.h>
@@ -528,13 +525,8 @@ std::string CodeTo(const char* tocode, const char* fromcode, std::basic_string_v
     while (src_bytes != 0)
     {
       size_t const iconv_result =
-#if defined(__NetBSD__)
-          iconv(conv_desc, reinterpret_cast<const char**>(&src_buffer), &src_bytes, &dst_buffer,
-                &dst_bytes);
-#else
           iconv(conv_desc, const_cast<char**>(reinterpret_cast<const char**>(&src_buffer)),
                 &src_bytes, &dst_buffer, &dst_bytes);
-#endif
       if ((size_t)-1 == iconv_result)
       {
         if (EILSEQ == errno || EINVAL == errno)

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"