main/postgresql: fix build on riscv64

see: https://www.postgresql.org/message-id/flat/20161119230312.GA8656%40redhat.com

main/postgresql/APKBUILD

@@ -117,6 +117,11 @@ build() {
 	export CFLAGS="${CFLAGS/-Os/-O2}"
 	export CPPFLAGS="${CPPFLAGS/-Os/-O2}"
 
+	# when disable-spinlocks is no longer required - check postgresql-bdr package
+	case $CARCH in
+		riscv64) local _configure=--disable-spinlocks;;
+	esac
+
 	./configure \
 		--build=$CBUILD \
 		--host=$CHOST \
@@ -132,7 +137,8 @@ build() {
 		--with-perl \
 		--with-python \
 		--with-icu \
-		--with-tcl
+		--with-tcl \
+		$_configure
 	make world
 }