package/libpqxx: update dependencies

As of v6.x, libpqxx takes advantage of thread support for transaction
sleeps (adding BR2_TOOLCHAIN_HAS_THREADS).

Re-order dependencies for:
 - Key feature (postgresql)
 - Architectures dependencies
 - Remaining dependencies

Fixes:

  http://autobuild.buildroot.net/results/e1d6f62edfc5eae90953f0a93fdb30d713409c37/

Signed-off-by: James Knight <james.knight@rockwellcollins.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

package/libpqxx/Config.in

@@ -1,14 +1,17 @@
 config BR2_PACKAGE_LIBPQXX
 	bool "libpqxx"
-	depends on BR2_INSTALL_LIBSTDCPP
-	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 # C++11
 	depends on BR2_PACKAGE_POSTGRESQL
+	depends on BR2_TOOLCHAIN_HAS_THREADS
+	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 # C++11
+	depends on BR2_INSTALL_LIBSTDCPP
 	help
 	  libpqxx is the official C++ client API for PostgreSQL, the
 	  enterprise-strength open-source relational database.
 
 	  http://pqxx.org/development/libpqxx/
 
-comment "libpqxx needs toolchain w/ C++ support, gcc >= 4.7"
-	depends on !BR2_INSTALL_LIBSTDCPP || !BR2_TOOLCHAIN_GCC_AT_LEAST_4_7
+comment "libpqxx needs toolchain w/ C++, gcc >= 4.7, threads"
 	depends on BR2_PACKAGE_POSTGRESQL
+	depends on !BR2_INSTALL_LIBSTDCPP || \
+		!BR2_TOOLCHAIN_GCC_AT_LEAST_4_7 || \
+		!BR2_TOOLCHAIN_HAS_THREADS