git: apply config.prefix to paths

from @michaelforney:
> It looks like all of those paths (`GIT_*_PATH`, `DEFAULT_GIT_TEMPLATE_DIR`)
> are used with the `system_path()` function, which prefixes them
> with `system_prefix()`.
>
> `system_prefix()` works in one of two ways, depending on whether
> RUNTIME_PREFIX is defined. If it is, it looks of the executable
> path using `/proc/self/exe`, finding the prefix dynamically similar
> to a relocatable gcc toolchain, otherwise it uses
> `FALLBACK_RUNTIME_PREFIX`.

see #15: Support configurable PREFIX

pkg/git/gen.lua

@@ -31,7 +31,7 @@ build('hooklist', '$outdir/hook-list.h', {
 	'|', '$srcdir/generate-hooklist.sh', '$srcdir/Documentation/githooks.txt'
 })
 
-cc('exec-cmd.c', nil, {cflags=[[$cflags '-DFALLBACK_RUNTIME_PREFIX=""']]})
+cc('exec-cmd.c', nil, {cflags=string.format([[$cflags '-DFALLBACK_RUNTIME_PREFIX="%s"']], config.prefix)})
 cc('common-main.c')
 cc('http.c')
 cc('compat/regex/regex.c', nil, {cflags='$cflags -DGAWK -DNO_MBSUPPORT'})