build: cleanup defining SYSCONFDIR on command line

Optimally, ./configure defines things in include/config.h. However,
that does not work well with defines that contain path names (see [1]).

Continue to use "-D" command line option. However, factor out the
definition to one place.

Also, rename from SYSCONFDIR to _NL_SYSCONFDIR_LIBNL.

Note that "src_lib_libnl_cli_3_la_CPPFLAGS" defined to
-DSYSCONFDIR=\"$(sysconfdir)\", without the "libnl" subdirectory. In
this case, the define was never used. The change doesn't matter.

[1] https://www.gnu.org/software/autoconf/manual/autoconf-2.63/html_node/Installation-Directory-Variables.html

Makefile.am

@@ -45,6 +45,10 @@ warn_cppflags = \
 	-Wno-unused-parameter \
 	$(NULL)
 
+defines_cppflags = \
+	-D_NL_SYSCONFDIR_LIBNL=\"$(sysconfdir)/libnl\" \
+	$(NULL)
+
 ###############################################################################
 
 libnlincludedir = $(includedir)/libnl@MAJ_VERSION@
@@ -391,8 +395,8 @@ default_includes = \
 
 lib_cppflags = \
 	$(warn_cppflags) \
+	$(defines_cppflags) \
 	-D_GNU_SOURCE \
-	-DSYSCONFDIR=\"$(sysconfdir)/libnl\" \
 	$(default_includes) \
 	$(NULL)
 
@@ -709,9 +713,9 @@ EXTRA_src_lib_libnl_cli_3_la_DEPENDENCIES = \
 	libnl-cli-3.sym
 src_lib_libnl_cli_3_la_CPPFLAGS = \
 	$(warn_cppflags) \
+	$(defines_cppflags) \
 	-D_GNU_SOURCE \
 	-DPKGLIBDIR=\"$(pkglibdir)\" \
-	-DSYSCONFDIR=\"$(sysconfdir)\" \
 	$(default_includes) \
 	$(NULL)
 src_lib_libnl_cli_3_la_LDFLAGS = \
@@ -729,8 +733,8 @@ src_lib_libnl_cli_3_la_LIBADD = \
 
 src_cppflags = \
 	$(warn_cppflags) \
+	$(defines_cppflags) \
 	-D_GNU_SOURCE \
-	-DSYSCONFDIR=\"$(sysconfdir)/libnl\" \
 	$(default_includes) \
 	$(NULL)
 
@@ -909,8 +913,8 @@ src_nl_util_addr_LDADD =            $(src_ldadd)
 
 tests_cppflags = \
 	$(warn_cppflags) \
+	$(defines_cppflags) \
 	-D_GNU_SOURCE \
-	-DSYSCONFDIR=\"$(sysconfdir)/libnl\" \
 	$(default_includes) \
 	$(NULL)
 

lib/route/nl-route.h

@@ -156,7 +156,7 @@ static inline int build_sysconf_path(char **strp, const char *filename)
 	sysconfdir = getenv("NLSYSCONFDIR");
 
 	if (!sysconfdir)
-		sysconfdir = SYSCONFDIR;
+		sysconfdir = _NL_SYSCONFDIR_LIBNL;
 
 	return asprintf(strp, "%s/%s", sysconfdir, filename);
 }