php: use TARGET_MAKE_ENV for buildconf

Otherwise we miss the PATH for hosts that lack autotools. Fixes:
http://autobuild.buildroot.net/results/368/368da962f657801817a4b962415e55606915c871/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>