php7-pecl-http: fix build when php5 is build in parallel (fixes#3149)
- remove hack to find required packages: when php5-pecl-{raphf,propro}
was built in paralle, then the hack could not determine which build
directory must be used; this was resolved by a previous commit, both
packages now install their header files in the staging area so that
this pecl module will find it properly
- refine build dir to avoid collision with (possible) php5 package variant
Signed-off-by: Michael Heimpold <mhei@heimpold.de>