fix compilation on EL7
    
    Git now requires C99 support and a zlib with uncompress2 by default.
    
    On EL7, gcc-4.8.5 requires a flag to enable C99 support.
    
    Compilation also fails without -fPIC on EL7, for reasons of which I am
    not entirely clear.  (I do not like making a change I cannot justify or
    explain properly, but it is better than dropping EL7 support until I
    have time to learn the reason(s).)
    
    Update the %build_cflags macro when building on EL7 to enable C99
    support and set -fPIC.
    
    Define NO_UNCOMPRESS2 to use compat/zlib-uncompress2.c.