Add optional systemd support for static builds

Implement conditional systemd linking to fix static build failures.
Static builds can now disable systemd with DISABLE_SYSTEMD=1.
Nix derivation updated with enableSystemd parameter for flexibility.
Provides backward compatibility while solving static linking issues.

Fixes: #348

Signed-off-by: Jindrich Novy <jnovy@redhat.com>

Makefile

@@ -38,10 +38,14 @@ override CFLAGS += $(shell $(PKG_CONFIG) --cflags glib-2.0) -DVERSION=\"$(VERSIO
 # "pkg-config --exists" will error if the package doesn't exist. Make can only compare
 # output of commands, so the echo commands are to allow pkg-config to error out, make to catch it,
 # and allow the compilation to complete.
+#
+# For static builds, systemd can be disabled with DISABLE_SYSTEMD=1
+ifneq ($(DISABLE_SYSTEMD), 1)
 ifeq ($(shell $(PKG_CONFIG) --exists libsystemd && echo "0"), 0)
 	override LIBS += $(shell $(PKG_CONFIG) --libs libsystemd)
 	override CFLAGS += $(shell $(PKG_CONFIG) --cflags libsystemd) -D USE_JOURNALD=1
 endif
+endif
 
 ifeq ($(shell hack/seccomp-notify.sh), 0)
 	override LIBS += $(shell $(PKG_CONFIG) --libs libseccomp) -ldl

nix/derivation.nix

@@ -1,5 +1,6 @@
 { stdenv
 , pkgs
+, enableSystemd ? false
 }:
 with pkgs; stdenv.mkDerivation rec {
   name = "conmon";
@@ -18,11 +19,15 @@ with pkgs; stdenv.mkDerivation rec {
   ] ++ [
     pkgsStatic.glib
     libseccomp
+  ] ++ lib.optionals enableSystemd [
+    # Only include systemd for dynamic builds, not static builds
+    # Static builds will use PKG_CONFIG_PATH approach instead
   ];
   prePatch = ''
     export CFLAGS='-static -pthread'
     export LDFLAGS='-s -w -static-libgcc -static'
     export EXTRA_LDFLAGS='-s -w -linkmode external -extldflags "-static -lm"'
+    ${lib.optionalString (!enableSystemd) "export DISABLE_SYSTEMD=1"}
   '';
   buildPhase = ''
     patchShebangs .