meson: Add user-provided c_args to bitcode_cflags

This is needed for example to pass an include path set in the CPPFLAGS
environment variable to the bitcode compile command.

Discussion: https://www.postgresql.org/message-id/flat/c1384a7b-ed12-4862-a0da-a05c7945171a%40eisentraut.org

src/backend/jit/llvm/meson.build

@@ -63,6 +63,7 @@ bitcode_cflags = ['-fno-strict-aliasing', '-fwrapv']
 if llvm.version().version_compare('=15.0')
   bitcode_cflags += ['-Xclang', '-no-opaque-pointers']
 endif
+bitcode_cflags += get_option('c_args')
 bitcode_cflags += cppflags
 
 # XXX: Worth improving on the logic to find directories here