gcc: support avr

- respect libc’s incdir and libdir
- make non-unix systems single threaded
- set LIMITS_H_TEST to false for avr
- misc updates to support new libc’s
- use multilib with avr

For threads we want to use:
- posix on unix systems
- win32 on windows
- single on everything else

For avr:
- add library directories for avrlibc
- to disable relro and bind
- avr5 should have precedence over avr3 - otherwise gcc uses the wrong one

pkgs/development/compilers/gcc/7/default.nix

@@ -67,7 +67,7 @@ let version = "7.3.0";
       [ "--with-as=${targetPackages.stdenv.cc.bintools}/bin/${targetPlatform.config}-as"
         "--with-ld=${targetPackages.stdenv.cc.bintools}/bin/${targetPlatform.config}-ld" ] ++
       (if crossMingw && crossStageStatic then [
-        "--with-headers=${libcCross}/include"
+        "--with-headers=${getDev libcCross}${libcCross.incdir or "/include"}"
         "--with-gcc"
         "--with-gnu-as"
         "--with-gnu-ld"
@@ -92,7 +92,7 @@ let version = "7.3.0";
         "--disable-libmpx" # requires libc
       ] else [
         (if crossDarwin then "--with-sysroot=${getLib libcCross}/share/sysroot"
-         else                "--with-headers=${getDev libcCross}/include")
+         else                "--with-headers=${getDev libcCross}${libcCross.incdir or "/include"}")
         "--enable-__cxa_atexit"
         "--enable-long-long"
       ] ++
@@ -115,11 +115,17 @@ let version = "7.3.0";
             "--disable-libgomp"
             # musl at least, disable: https://git.buildroot.net/buildroot/commit/?id=873d4019f7fb00f6a80592224236b3ba7d657865
             "--disable-libmpx"
-          ] ++ [
-          "--enable-threads=posix"
-          "--enable-nls"
-          "--disable-decimal-float" # No final libdecnumber (it may work only in 386)
-        ]));
+          ]
+          ++ optional (targetPlatform.libc == "newlib") "--with-newlib"
+          ++ optional (targetPlatform.libc == "avrlibc") "--with-avrlibc"
+          ++ [
+            "--enable-threads=${if targetPlatform.isUnix then "posix"
+                                else if targetPlatform.isWindows then "win32"
+                                else "single"}"
+            "--enable-nls"
+            # No final libdecnumber (it may work only in 386)
+            "--disable-decimal-float"
+          ]));
     stageNameAddon = if crossStageStatic then "-stage-static" else "-stage-final";
     crossNameAddon = if targetPlatform != hostPlatform then "${targetPlatform.config}${stageNameAddon}-" else "";
 
@@ -188,7 +194,12 @@ stdenv.mkDerivation ({
             sed -i gcc/config/linux.h -e '1i#undef LOCAL_INCLUDE_DIR'
         ''
         )
-    else "");
+    else "")
+      + stdenv.lib.optionalString targetPlatform.isAvr ''
+        makeFlagsArray+=(
+           'LIMITS_H_TEST=false'
+        )
+      '';
 
   inherit noSysDirs staticCompiler crossStageStatic
     libcCross crossMingw;
@@ -267,7 +278,7 @@ stdenv.mkDerivation ({
       }"
     ] ++
 
-    (if enableMultilib
+    (if (enableMultilib || targetPlatform.isAvr)
       then ["--enable-multilib" "--disable-libquadmath"]
       else ["--disable-multilib"]) ++
     optional (!enableShared) "--disable-shared" ++
@@ -334,20 +345,20 @@ stdenv.mkDerivation ({
   EXTRA_TARGET_FLAGS = optionals
     (targetPlatform != hostPlatform && libcCross != null)
     ([
-      "-idirafter ${getDev libcCross}/include"
+      "-idirafter ${getDev libcCross}${libcCross.incdir or "/include"}"
     ] ++ optionals (! crossStageStatic) [
-      "-B${libcCross.out}/lib"
+      "-B${libcCross.out}${libcCross.libdir or "/lib"}"
     ]);
 
   EXTRA_TARGET_LDFLAGS = optionals
     (targetPlatform != hostPlatform && libcCross != null)
     ([
-      "-Wl,-L${libcCross.out}/lib"
+      "-Wl,-L${libcCross.out}${libcCross.libdir or "/lib"}"
     ] ++ (if crossStageStatic then [
-        "-B${libcCross.out}/lib"
+        "-B${libcCross.out}${libcCross.libdir or "/lib"}"
       ] else [
-        "-Wl,-rpath,${libcCross.out}/lib"
-        "-Wl,-rpath-link,${libcCross.out}/lib"
+        "-Wl,-rpath,${libcCross.out}${libcCross.libdir or "/lib"}"
+        "-Wl,-rpath-link,${libcCross.out}${libcCross.libdir or "/lib"}"
     ]));
 
   passthru =