Switching from php5 to php and removing unneeded variable in Makefile

There's no reason this should be specific to PHP version 5, plus
'php5' is not always registered as an alias to php. For example,
I'm running PHP 5.4.24, but 'php5' does nothing in my environment.

Removing MW_MF_INSTALL_PATH. We already have MW_INSTALL_PATH and
extensions are always in the extension dir, so I don't think we
need to create a separate variable for this.

Change-Id: I863ced57532cc4c41afdadc1e8aaa2e3eb605455
Merge "Switching from php5 to php and removing unneeded variable in Makefile"