Fix pothos build

pkgs/applications/radio/pothos/cstring.patch

@@ -0,0 +1,50 @@
+Submodule blocks contains modified content
+diff --git a/blocks/file/BinaryFileSink.cpp b/blocks/file/BinaryFileSink.cpp
+index 31c9a41..0083b0d 100644
+--- a/blocks/file/BinaryFileSink.cpp
++++ b/blocks/file/BinaryFileSink.cpp
+@@ -13,6 +13,7 @@
+ #endif //_MSC_VER
+ #include <stdio.h>
+ #include <cerrno>
++#include <cstring>
+
+ #ifndef O_BINARY
+ #define O_BINARY 0
+diff --git a/blocks/file/BinaryFileSource.cpp b/blocks/file/BinaryFileSource.cpp
+index 0151231..379d383 100644
+--- a/blocks/file/BinaryFileSource.cpp
++++ b/blocks/file/BinaryFileSource.cpp
+@@ -13,6 +13,7 @@
+ #endif //_MSC_VER
+ #include <stdio.h>
+ #include <cerrno>
++#include <cstring>
+
+ #ifndef O_BINARY
+ #define O_BINARY 0
+diff --git a/blocks/file/TextFileSink.cpp b/blocks/file/TextFileSink.cpp
+index b4b2f08..2be66e2 100644
+--- a/blocks/file/TextFileSink.cpp
++++ b/blocks/file/TextFileSink.cpp
+@@ -6,6 +6,7 @@
+ #include <complex>
+ #include <fstream>
+ #include <cerrno>
++#include <cstring>
+
+ /***********************************************************************
+  * |PothosDoc Text File Sink
+Submodule soapy contains modified content
+diff --git a/soapy/DemoController.cpp b/soapy/DemoController.cpp
+index 4ce8ead..9a4e742 100644
+--- a/soapy/DemoController.cpp
++++ b/soapy/DemoController.cpp
+@@ -6,6 +6,7 @@
+ #include <iostream>
+ #include <chrono>
+ #include <algorithm> //min/max
++#include <cstring>
+
+ /***********************************************************************
+  * |PothosDoc SDR Demo Controller

pkgs/applications/radio/pothos/default.nix

@@ -41,8 +41,13 @@ mkDerivation rec {
       url = "https://github.com/pothosware/PothosCore/commit/092d1209b0fd0aa8a1733706c994fa95e66fd017.patch";
       hash = "sha256-bZXG8kD4+1LgDV8viZrJ/DMjg8UvW7b5keJQDXurfkA=";
     })
+    # various source files are missing imports of <cstring>
+    ./cstring.patch
   ];
 
+  # poco 1.14 requires c++17
+  NIX_CFLAGS_COMPILE = [ "-std=gnu++17" ];
+
   nativeBuildInputs = [
     cmake
     pkg-config