[meson] add icu DEFS required for compilation

In some cases we need to add additionl defs to build against icu if
icu has certain options configured.

ICU warns about this when building:

*** WARNING: You must set the following flags before code compiled against this ICU will function properly:

    -DU_DISABLE_RENAMING=1

We can fetch these flags from the icu pkgconfig and add them if
required.

This fixes symbol errors if ICU is built without renaming.

meson.build

@@ -123,6 +123,13 @@ if not get_option('icu').disabled()
   endif
 endif
 
+if icu_dep.found()
+  icu_defs = icu_dep.get_variable(pkgconfig: 'DEFS', default_value: '')
+  if icu_defs != ''
+    add_project_arguments(icu_defs, language: ['c', 'cpp'])
+  endif
+endif
+
 cairo_dep = null_dep
 cairo_ft_dep = null_dep
 if not get_option('cairo').disabled()