Only link with libdl on Linux

Linux is (as far as I know) the only mainstream operating system that
requires linking with libdl for dlopen.  On BSD, libdl doesn't exist,
so on non-FreeBSD BSDs linking will currently fail.  On macOS, it's
apparently just a symlink to libSystem (macOS libc), presumably
present for compatibility with things that assume Linux.

So the right thing to do here is to only add -ldl on Linux, not to add
it for everything that isn't FreeBSD.

nix-rust/local.mk

@@ -8,8 +8,13 @@ endif
 
 libnixrust_PATH := $(d)/target/$(RUST_DIR)/libnixrust.$(SO_EXT)
 libnixrust_INSTALL_PATH := $(libdir)/libnixrust.$(SO_EXT)
-libnixrust_LDFLAGS_USE := -L$(d)/target/$(RUST_DIR) -lnixrust -ldl
-libnixrust_LDFLAGS_USE_INSTALLED := -L$(libdir) -lnixrust -ldl
+libnixrust_LDFLAGS_USE := -L$(d)/target/$(RUST_DIR) -lnixrust
+libnixrust_LDFLAGS_USE_INSTALLED := -L$(libdir) -lnixrust
+
+ifeq ($(OS), Linux)
+libnixrust_LDFLAGS_USE += -ldl
+libnixrust_LDFLAGS_USE_INSTALLED += -ldl
+endif
 
 ifeq ($(OS), Darwin)
 libnixrust_BUILD_FLAGS = NIX_LDFLAGS="-undefined dynamic_lookup"

src/libexpr/local.mk

@@ -16,7 +16,7 @@ libexpr_CXXFLAGS += -I src/libutil -I src/libstore -I src/libfetchers -I src/lib
 libexpr_LIBS = libutil libstore libfetchers
 
 libexpr_LDFLAGS = -lboost_context
-ifneq ($(OS), FreeBSD)
+ifeq ($(OS), Linux)
  libexpr_LDFLAGS += -ldl
 endif
 

src/libstore/local.mk

@@ -9,7 +9,7 @@ libstore_SOURCES := $(wildcard $(d)/*.cc $(d)/builtins/*.cc $(d)/build/*.cc)
 libstore_LIBS = libutil
 
 libstore_LDFLAGS = $(SQLITE3_LIBS) -lbz2 $(LIBCURL_LIBS) $(SODIUM_LIBS) -pthread
-ifneq ($(OS), FreeBSD)
+ifeq ($(OS), Linux)
  libstore_LDFLAGS += -ldl
 endif