grep: make sure NO_LIBPCRE1_JIT disable JIT in PCRE1

e87de7cab4 ("grep: un-break building with PCRE < 8.32", 2017-05-25)
added a restriction for JIT support that is no longer needed after
pcre_jit_exec() calls were removed.

Reorganize the definitions in grep.h so that JIT support could be
detected early and NO_LIBPCRE1_JIT could be used reliably to enforce
JIT doesn't get used.

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

Makefile

@@ -34,13 +34,8 @@ all::
 # library. Support for version 1 will likely be removed in some future
 # release of Git, as upstream has all but abandoned it.
 #
-# When using USE_LIBPCRE1, define NO_LIBPCRE1_JIT if the PCRE v1
-# library is compiled without --enable-jit. We will auto-detect
-# whether the version of the PCRE v1 library in use has JIT support at
-# all, but we unfortunately can't auto-detect whether JIT support
-# hasn't been compiled in in an otherwise JIT-supporting version. If
-# you have link-time errors about a missing `pcre_jit_exec` define
-# this, or recompile PCRE v1 with --enable-jit.
+# When using USE_LIBPCRE1, define NO_LIBPCRE1_JIT if you want to
+# disable JIT even if supported by your library.
 #
 # Define LIBPCREDIR=/foo/bar if your PCRE header and library files are
 # in /foo/bar/include and /foo/bar/lib directories. Which version of

grep.h

@@ -3,14 +3,12 @@
 #include "color.h"
 #ifdef USE_LIBPCRE1
 #include <pcre.h>
-#ifdef PCRE_CONFIG_JIT
-#if PCRE_MAJOR >= 8 && PCRE_MINOR >= 32
 #ifndef NO_LIBPCRE1_JIT
+#ifdef PCRE_CONFIG_JIT
 #define GIT_PCRE1_USE_JIT
 #define GIT_PCRE_STUDY_JIT_COMPILE PCRE_STUDY_JIT_COMPILE
 #endif
 #endif
-#endif
 #ifndef GIT_PCRE_STUDY_JIT_COMPILE
 #define GIT_PCRE_STUDY_JIT_COMPILE 0
 #endif