netbsd: allow replacing nbcompat with libbsd

pkgs/os-specific/bsd/netbsd/compat.patch

@@ -0,0 +1,18 @@
+diff -u -r1.35.2.1 nbtool_config.h.in
+--- a/nbtool_config.h.in	22 Apr 2015 07:18:58 -0000	1.35.2.1
++++ b/nbtool_config.h.in	31 May 2018 01:46:53 -0000
+@@ -680,5 +680,14 @@
+ /* Define if you have u_int8_t, but not uint8_t. */
+ #undef uint8_t
+ 
++#ifdef __cplusplus
++extern "C" {
++#endif
++
+ #include "compat_defs.h"
++
++#ifdef __cplusplus
++}
++#endif
++
+ #endif /* !__NETBSD_NBTOOL_CONFIG_H__ */

pkgs/os-specific/bsd/netbsd/default.nix

@@ -162,7 +162,7 @@ let
     extraPaths = [ make.src ] ++ make.extraPaths;
   };
 
-  compat = netBSDDerivation {
+  compat = netBSDDerivation rec {
     path = "tools/compat";
     sha256 = "17phkfafybxwhzng44k5bhmag6i55br53ky1nwcmw583kg2fa86z";
     version = "7.1.2";
@@ -182,6 +182,8 @@ let
     installFlags = [];
     RENAME = "-D";
 
+    patches = [ ./compat.patch ];
+
     postInstall = ''
       mv $out/include/compat/* $out/include
       rmdir $out/include/compat
@@ -204,6 +206,11 @@ let
       install -D $NETBSDSRCDIR/include/utmpx.h $out/include/utmpx.h
       install -D $NETBSDSRCDIR/include/tzfile.h $out/include/tzfile.h
 
+      mkdir -p $out/lib/pkgconfig
+      substitute ${./libbsd-overlay.pc} $out/lib/pkgconfig/libbsd-overlay.pc \
+        --subst-var-by out $out \
+        --subst-var-by version ${version}
+
       # Remove lingering /usr references
       if [ -d $out/usr ]; then
         cd $out/usr

pkgs/os-specific/bsd/netbsd/libbsd-overlay.pc

@@ -0,0 +1,11 @@
+prefix=@out@
+exec_prefix=${prefix}
+libdir=${exec_prefix}/lib
+includedir=${prefix}/include
+
+Name: nbcompat
+Description: NetBSD compatibility framework
+Version: @version@
+URL: https://www.netbsd.org/
+Libs: -L${libdir} -lnbcompat
+Cflags: -I${includedir} -DHAVE_NBTOOL_CONFIG_H -include nbtool_config.h