meson: fix exec path with enabled runtime prefix

When the runtime prefix option is enabled, Git is built such that it
knows to locate its binaries relative to the directory a binary is being
executed from. This requires us to figure out relative paths, which is
handled in `system_prefix()` by trying to strip a couple of well-known
paths.

One of these paths, GIT_EXEC_PATH, is expected to be absolute when
runtime prefixes are enabled, but relative otherwise. And while our
Makefile gets this correctly, in Meson we always wire up the absolute
path, which may result in us not being able to find binaries.

Fix this by conditionally injecting the paths depending on whether or
not the `runtime_prefix` option is enabled.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

meson.build

@@ -691,7 +691,6 @@ libgit_c_args = [
   '-DETC_GITATTRIBUTES="' + get_option('gitattributes') + '"',
   '-DETC_GITCONFIG="' + get_option('gitconfig') + '"',
   '-DFALLBACK_RUNTIME_PREFIX="' + get_option('prefix') + '"',
-  '-DGIT_EXEC_PATH="' + get_option('prefix') / get_option('libexecdir') / 'git-core"',
   '-DGIT_HOST_CPU="' + host_machine.cpu_family() + '"',
   '-DGIT_HTML_PATH="' + get_option('datadir') / 'doc/git-doc"',
   '-DGIT_INFO_PATH="' + get_option('infodir') + '"',
@@ -1466,6 +1465,7 @@ endif
 if get_option('runtime_prefix')
   libgit_c_args += '-DRUNTIME_PREFIX'
   build_options_config.set('RUNTIME_PREFIX', 'true')
+  git_exec_path = get_option('libexecdir') / 'git-core'
 
   if compiler.has_header('mach-o/dyld.h')
     libgit_c_args += '-DHAVE_NS_GET_EXECUTABLE_PATH'
@@ -1502,7 +1502,9 @@ if get_option('runtime_prefix')
   endif
 else
   build_options_config.set('RUNTIME_PREFIX', 'false')
+  git_exec_path = get_option('prefix') / get_option('libexecdir') / 'git-core'
 endif
+libgit_c_args += '-DGIT_EXEC_PATH="' + git_exec_path + '"'
 
 git_version_file = custom_target(
   command: [