fix building with Musl, fixing static builds

Musl stdout macro expands¹ to something that isn't a valid identifier,
so we get syntax errors when compiling usage of a method called stdout
with Musl's stdio.h.

[1]: https://git.musl-libc.org/cgit/musl/tree/include/stdio.h?id=ab31e9d6a0fa7c5c408856c89df2dfb12c344039#n67

Change-Id: I10e6f6a49504399bf8edd59c5d9e4e62449469e8

src/libfetchers/git.cc

@@ -695,7 +695,7 @@ struct GitInputScheme : InputScheme
             });
             Finally const _wait([&] { proc.wait(); });
 
-            unpackTarfile(*proc.stdout(), tmpDir);
+            unpackTarfile(*proc.getStdout(), tmpDir);
         }
 
         auto storePath = store->addToStore(name, tmpDir, FileIngestionMethod::Recursive, htSHA256, filter);

src/libstore/build/derivation-goal.cc

@@ -932,7 +932,7 @@ void runPostBuildHook(
     Finally const _wait([&] { proc.wait(); });
 
     // FIXME just process the data, without a wrapper sink class
-    proc.stdout()->drainInto(sink);
+    proc.getStdout()->drainInto(sink);
 }
 
 void DerivationGoal::buildDone()

src/libutil/processes.cc

@@ -251,7 +251,7 @@ std::pair<int, std::string> runProgram(RunOptions && options)
     try {
         auto proc = runProgram2(options);
         Finally const _wait([&] { proc.wait(); });
-        stdout = proc.stdout()->drain();
+        stdout = proc.getStdout()->drain();
     } catch (ExecError & e) {
         status = e.status;
     }

src/libutil/processes.hh

@@ -106,7 +106,7 @@ public:
 
     void wait();
 
-    Source * stdout() const { return stdoutSource.get(); }
+    Source * getStdout() const { return stdoutSource.get(); };
 };
 
 std::pair<int, std::string> runProgram(RunOptions && options);