man-pages: respect api-documentation

* let manpages.bbclass to enable manpages PACKAGECONFIG based on
api-documentation DISTRO_FEATURES
PACKAGECONFIG_append_class-target = " ${@bb.utils.contains('DISTRO_FEATURES', 'api-documentation', 'manpages','', d)}"
* it's true that building man-pages without manpages being enabled
doesn't make much sense, but it's included through couple
packagegroups:
meta/recipes-core/packagegroups/packagegroup-self-hosted.bb: man-pages \
meta/recipes-extended/packagegroups/packagegroup-core-lsb.bb: man-pages \
or in world even for people who might not be interested
in man-pages

---
 meta/recipes-extended/man-pages/man-pages_4.16.bb | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

index 0fa124fe7c..a64587cf0b 100644
@@ -14,8 +14,7 @@ inherit manpages
 
 MAN_PKG = "${PN}"
 
-# help manpages.bbclass deal with RDEPNDS and postinstall scripts
-PACKAGECONFIG = "manpages"
+PACKAGECONFIG ??= ""
 PACKAGECONFIG[manpages] = ""
 
 do_configure[noexec] = "1"

2.17.1