grep: refactor and simplify PCRE1 support

grep: refactor and simplify PCRE1 support

The code used both a macro and a variable to keep track if JIT
support was desired and relied on the fact that a non JIT
enabled library will ignore a request for JIT compilation
(as defined by the second parameter of the call to pcre_study)

Cleanup the multiple levels of macros used and call pcre_study
with the right parameter after JIT support has been confirmed
and unless it was requested to be disabled with NO_LIBPCRE1_JIT

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>