build: fix linking to libsodium on macOS

Currently on I see link failures like the following:
```bash
./configure
<snip>
checking for libsodium with IETF chacha20 variants... yes
<snip>
ar libccan.a
ld ccan/ccan/cdump/tools/cdump-enumstr
ld: library 'sodium' not found
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

The configure check passes under Clang here.

Changelog-Fixed: build: fix linking against libsodium on macOS.

Makefile

@@ -263,7 +263,7 @@ LIBRARY_PATH := /usr/local/lib
 endif
 
 CPPFLAGS += -DCLN_NEXT_VERSION="\"$(CLN_NEXT_VERSION)\"" -DPKGLIBEXECDIR="\"$(pkglibexecdir)\"" -DBINDIR="\"$(bindir)\"" -DPLUGINDIR="\"$(plugindir)\"" -DCCAN_TAL_NEVER_RETURN_NULL=1
-CFLAGS = $(CPPFLAGS) $(CWARNFLAGS) $(CDEBUGFLAGS) $(COPTFLAGS) -I $(CCANDIR) $(EXTERNAL_INCLUDE_FLAGS) -I . -I$(CPATH) $(SQLITE3_CFLAGS) $(POSTGRES_INCLUDE) $(FEATURES) $(COVFLAGS) $(DEV_CFLAGS) -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS $(PIE_CFLAGS) $(COMPAT_CFLAGS) $(CSANFLAGS)
+CFLAGS = $(CPPFLAGS) $(CWARNFLAGS) $(CDEBUGFLAGS) $(COPTFLAGS) -I $(CCANDIR) $(EXTERNAL_INCLUDE_FLAGS) -I . -I$(CPATH) $(SQLITE3_CFLAGS) $(SODIUM_CFLAGS) $(POSTGRES_INCLUDE) $(FEATURES) $(COVFLAGS) $(DEV_CFLAGS) -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS $(PIE_CFLAGS) $(COMPAT_CFLAGS) $(CSANFLAGS)
 
 # If CFLAGS is already set in the environment of make (to whatever value, it
 # does not matter) then it would export it to subprocesses with the above value

configure

@@ -355,6 +355,13 @@ if command -v "${PKG_CONFIG}" >/dev/null; then
     SQLITE3_LDLIBS="$("${PKG_CONFIG}" --silence-errors --libs sqlite3 || :)"
 fi
 
+SODIUM_CFLAGS=""
+SODIUM_LDLIBS="-lsodium"
+if command -v "${PKG_CONFIG}" >/dev/null; then
+    SODIUM_CFLAGS="$("${PKG_CONFIG}" --silence-errors --cflags libsodium || :)"
+    SODIUM_LDLIBS="$("${PKG_CONFIG}" --silence-errors --libs libsodium || :)"
+fi
+
 POSTGRES_INCLUDE=""
 POSTGRES_LDLIBS=""
 if command -v "${PG_CONFIG}" >/dev/null; then
@@ -365,7 +372,7 @@ fi
 # Clean up on exit.
 trap "rm -f $CONFIG_VAR_FILE.$$" 0
 
-$CONFIGURATOR --extra-tests --autotools-style --var-file=$CONFIG_VAR_FILE.$$ --header-file=$CONFIG_HEADER.$$ --configurator-cc="$CONFIGURATOR_CC" --wrapper="$CONFIGURATOR_WRAPPER" "$CC" ${CWARNFLAGS-$BASE_WARNFLAGS} $CDEBUGFLAGS $COPTFLAGS $CSANFLAGS -I$CPATH -L$LIBRARY_PATH $SQLITE3_CFLAGS $POSTGRES_INCLUDE <<EOF
+$CONFIGURATOR --extra-tests --autotools-style --var-file=$CONFIG_VAR_FILE.$$ --header-file=$CONFIG_HEADER.$$ --configurator-cc="$CONFIGURATOR_CC" --wrapper="$CONFIGURATOR_WRAPPER" "$CC" ${CWARNFLAGS-$BASE_WARNFLAGS} $CDEBUGFLAGS $COPTFLAGS $CSANFLAGS -I$CPATH -L$LIBRARY_PATH $SQLITE3_CFLAGS $SODIUM_CFLAGS $POSTGRES_INCLUDE <<EOF
 
 var=HAVE_ZLIB
 desc=zlib support
@@ -388,7 +395,7 @@ int main(void)
 var=HAVE_GOOD_LIBSODIUM
 desc=libsodium with IETF chacha20 variants
 style=DEFINES_EVERYTHING|EXECUTE|MAY_NOT_COMPILE
-link=-lsodium
+link=$SODIUM_LDLIBS
 code=
 #include <sodium.h>
 #include <stdio.h>
@@ -552,6 +559,8 @@ add_var SQLITE3_CFLAGS "$SQLITE3_CFLAGS"
 add_var SQLITE3_LDLIBS "$SQLITE3_LDLIBS"
 add_var POSTGRES_INCLUDE "$POSTGRES_INCLUDE"
 add_var POSTGRES_LDLIBS "$POSTGRES_LDLIBS"
+add_var SODIUM_CFLAGS "$SODIUM_CFLAGS"
+add_var SODIUM_LDLIBS "$SODIUM_LDLIBS"
 add_var VALGRIND "$VALGRIND"
 add_var DEBUGBUILD "$DEBUGBUILD"
 add_var COMPAT "$COMPAT" $CONFIG_HEADER.$$

external/Makefile

@@ -50,7 +50,7 @@ EXTERNAL_INCLUDE_FLAGS += -I external/libsodium/src/libsodium/include \
 			  -I $(TARGET_DIR)/libsodium-build/src/libsodium/include
 EXTERNAL_LIBS += ${TARGET_DIR}/libsodium.a
 else
-LDLIBS += -lsodium
+LDLIBS += $(SODIUM_LDLIBS)
 endif
 
 ifeq ($(HAVE_ZLIB),1)