abuild: add depends_libs variable to default_libs()

Other subpackage such as -dev, -doc and even -openrc allow adjusting
depends of the subpackage through such a variable. This is, for
instance, useful to remove a dependency of the origin package from the
-libs subpackage.

While at it document it in APKBUILD(5).

APKBUILD.5

@@ -404,6 +404,8 @@ variables.
 .Bl -tag -width Ds
 .It Cm depends_dev
 Specifies the run-time dependencies of the -dev subpackage.
+.It Cm depends_libs
+Specifies the run-time dependencies of the -libs subpackage.
 .It Cm giturl
 Specifies the URL of the Git repository to use with
 .Cm abuild checkout .

abuild.in

@@ -1731,6 +1731,7 @@ dev() {
 
 # predefined splitfunc libs
 default_libs() {
+	depends="$depends_libs"
 	pkgdesc="$pkgdesc (libraries)"
 	local dir= file=
 	for dir in lib usr/lib; do