libserial: prevent documentation from being build

If docbook2pdf is present on the host, the documentation is built, no
matter all the --disable-doc*
But forcing ac_cv_prog_DOCBOOK2PDF to no tells configure that
docbook2pdf is absent, and thus the documentation is not built.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/libserial/libserial.mk

@@ -10,4 +10,6 @@ LIBSERIAL_INSTALL_STAGING = YES
 LIBSERIAL_LICENSE = GPLv2+
 LIBSERIAL_LICENSE_FILES = COPYING
 
+LIBSERIAL_CONF_ENV = ac_cv_prog_DOCBOOK2PDF=no
+
 $(eval $(autotools-package))