diff --git a/local.mk b/local.mk
index dc10e6870..0a2254237 100644
--- a/local.mk
+++ b/local.mk
@@ -7,8 +7,7 @@ dist-files += configure config.h.in nix.spec perl/configure
 
 clean-files += Makefile.config
 
-GLOBAL_CXXFLAGS += -I . -I src -I src/libutil -I src/libstore -I src/libmain -I src/libexpr \
-  -Wno-unneeded-internal-declaration
+GLOBAL_CXXFLAGS += -I . -I src -I src/libutil -I src/libstore -I src/libmain -I src/libexpr
 
 $(foreach i, config.h $(call rwildcard, src/lib*, *.hh), \
   $(eval $(call install-file-in, $(i), $(includedir)/nix, 0644)))
diff --git a/src/boost/format/feed_args.hpp b/src/boost/format/feed_args.hpp
index 3d0b47b4a..cdd57fdf2 100644
--- a/src/boost/format/feed_args.hpp
+++ b/src/boost/format/feed_args.hpp
@@ -37,6 +37,13 @@ namespace  {
     os.str(emptyStr); 
   }
 
+  void do_pad( std::string & s, 
+                std::streamsize w, 
+                const char c, 
+                std::ios::fmtflags f, 
+                bool center)
+    __attribute__ ((unused));
+
   void do_pad( std::string & s, 
                 std::streamsize w, 
                 const char c, 
diff --git a/src/libexpr/eval.cc b/src/libexpr/eval.cc
index f6cdedb37..5e1ae63c4 100644
--- a/src/libexpr/eval.cc
+++ b/src/libexpr/eval.cc
@@ -202,7 +202,7 @@ void initGC()
 
     GC_INIT();
 
-    GC_oom_fn = oomHandler;
+    GC_set_oom_fn(oomHandler);
 
     /* Set the initial heap size to something fairly big (25% of
        physical RAM, up to a maximum of 384 MiB) so that in most cases
diff --git a/src/libexpr/local.mk b/src/libexpr/local.mk
index 620050a13..daa3258f0 100644
--- a/src/libexpr/local.mk
+++ b/src/libexpr/local.mk
@@ -6,8 +6,6 @@ libexpr_DIR := $(d)
 
 libexpr_SOURCES := $(wildcard $(d)/*.cc) $(wildcard $(d)/primops/*.cc) $(d)/lexer-tab.cc $(d)/parser-tab.cc
 
-libexpr_CXXFLAGS := -Wno-deprecated-register
-
 libexpr_LIBS = libutil libstore libformat
 
 libexpr_LDFLAGS =
diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index 33c9e3704..9bf1ab5aa 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -3376,10 +3376,10 @@ void SubstitutionGoal::tryToRun()
     trace("trying to run");
 
     /* Make sure that we are allowed to start a build.  Note that even
-       is maxBuildJobs == 0 (no local builds allowed), we still allow
+       if maxBuildJobs == 0 (no local builds allowed), we still allow
        a substituter to run.  This is because substitutions cannot be
        distributed to another machine via the build hook. */
-    if (worker.getNrLocalBuilds() >= (settings.maxBuildJobs == 0 ? 1 : settings.maxBuildJobs)) {
+    if (worker.getNrLocalBuilds() >= std::min(1U, (unsigned int) settings.maxBuildJobs)) {
         worker.waitForBuildSlot(shared_from_this());
         return;
     }
@@ -3660,7 +3660,7 @@ void Worker::run(const Goals & _topGoals)
         if (!children.empty() || !waitingForAWhile.empty())
             waitForInput();
         else {
-            if (awake.empty() && settings.maxBuildJobs == 0) throw Error(
+            if (awake.empty() && 0 == settings.maxBuildJobs) throw Error(
                 "unable to start any build; either increase ‘--max-jobs’ "
                 "or enable distributed builds");
             assert(!awake.empty());
@@ -3697,9 +3697,9 @@ void Worker::waitForInput()
     auto nearest = steady_time_point::max(); // nearest deadline
     for (auto & i : children) {
         if (!i.respectTimeouts) continue;
-        if (settings.maxSilentTime != 0)
+        if (0 != settings.maxSilentTime)
             nearest = std::min(nearest, i.lastOutput + std::chrono::seconds(settings.maxSilentTime));
-        if (settings.buildTimeout != 0)
+        if (0 != settings.buildTimeout)
             nearest = std::min(nearest, i.timeStarted + std::chrono::seconds(settings.buildTimeout));
     }
     if (nearest != steady_time_point::max()) {
@@ -3777,7 +3777,7 @@ void Worker::waitForInput()
         }
 
         if (goal->getExitCode() == Goal::ecBusy &&
-            settings.maxSilentTime != 0 &&
+            0 != settings.maxSilentTime &&
             j->respectTimeouts &&
             after - j->lastOutput >= std::chrono::seconds(settings.maxSilentTime))
         {
@@ -3788,7 +3788,7 @@ void Worker::waitForInput()
         }
 
         else if (goal->getExitCode() == Goal::ecBusy &&
-            settings.buildTimeout != 0 &&
+            0 != settings.buildTimeout &&
             j->respectTimeouts &&
             after - j->timeStarted >= std::chrono::seconds(settings.buildTimeout))
         {
diff --git a/src/libstore/download.cc b/src/libstore/download.cc
index d073e870b..42fd05bd8 100644
--- a/src/libstore/download.cc
+++ b/src/libstore/download.cc
@@ -369,7 +369,7 @@ struct CurlDownloader : public Downloader
         curl_multi_setopt(curlm, CURLMOPT_PIPELINING, CURLPIPE_MULTIPLEX);
         #endif
         curl_multi_setopt(curlm, CURLMOPT_MAX_TOTAL_CONNECTIONS,
-            settings.binaryCachesParallelConnections);
+            settings.binaryCachesParallelConnections.get());
 
         enableHttp2 = settings.enableHttp2;