Bug 1635273 - Make FT_LCD_FILTER_H a relative rather than absolute pa…

…th. r=froydnj

This helps get sccache hits across source directories.

The previous presentation guaranteed the version of `ftlcdfil.h` in
the tree, while this unorthodox presentation only ensures that the
`freetype2` directory has parent `modules`.  This is at least unlikely
to occur in random `/usr/include` directories.

If this turns out to be an issue it would be possible to copy
`ftlcdfil.h` into the object directory, potentially with a unique
name, and then reference that (with a relative path).

Differential Revision: https://phabricator.services.mozilla.com/D73759

gfx/cairo/cairo/src/moz.build

@@ -207,7 +207,7 @@ DEFINES['HAVE_UINT64_T'] = True
 
 if CONFIG['MOZ_TREE_FREETYPE']:
     DEFINES['HAVE_FT_LIBRARY_SETLCDFILTER'] = True
-    DEFINES['FT_LCD_FILTER_H'] = '%s/modules/freetype2/include/freetype/ftlcdfil.h' % TOPSRCDIR
+    DEFINES['FT_LCD_FILTER_H'] = '../../../modules/freetype2/include/freetype/ftlcdfil.h'
 
 # Suppress warnings in third-party code.
 CFLAGS += [