[build] No need to pass source directory to gen-hb-version

The input file is by definition in the source directory, so dirname()
that instead of needing the directory to be passed.

Needed because a follow-up commit will change when this is called, and the
source directory isn't trivially available at that point.

src/gen-hb-version.py

@@ -4,15 +4,15 @@
 
 import os, sys, shutil
 
-if len (sys.argv) < 5:
+if len (sys.argv) < 4:
 	sys.exit(__doc__)
 
 version = sys.argv[1]
 major, minor, micro = version.split (".")
 
 OUTPUT = sys.argv[2]
-CURRENT_SOURCE_DIR = sys.argv[3]
-INPUT = sys.argv[4]
+INPUT = sys.argv[3]
+CURRENT_SOURCE_DIR = os.path.dirname(INPUT)
 
 with open (INPUT, "r", encoding='utf-8') as template:
 	with open (OUTPUT, "wb") as output:

src/meson.build

@@ -286,7 +286,7 @@ custom_target('hb-version.h',
   input: 'hb-version.h.in',
   output: 'hb-version.h',
   command: [find_program('gen-hb-version.py'), meson.project_version(),
-            '@OUTPUT@', '@CURRENT_SOURCE_DIR@', '@INPUT@'],
+            '@OUTPUT@', '@INPUT@'],
 )
 
 ragel = find_program('ragel', required: false)