This patch implements metadirective parsing in the Fortran frontend.

The code previously used to process context selectors in 'declare 
variant' is refactored so that it can be reused in metadirectives. The 
big case lists in parse_executable are moved into macros, since 
parse_omp_metadirective_body needs to know how to act depending on the 
type of directive variant. The selection of end statements in 
parse_omp_do and parse_omp_structured_block are also delegated to 
gfc_omp_end_stmt.

Labels in directive variant bodies are handled by assigning a unique 
number to each statement body parsed in a metadirective, and adding this 
number as a field to gfc_st_label, such that labels with identical 
numbers but different region ids are considered different.

I have also reverted my previous changes to the TREE_STRING_LENGTH check 
in omp_check_context_selector and omp_context_name_list_prop. This is 
because in the accel compiler, lang_GNU_Fortran returns 0 even when the 
code is in Fortran, resulting in the selector failing to match. Instead, 
I opted to increment the TREE_STRING_LENGTH when it is created in 
gfc_trans_omp_set_selector - this should be safe as it is an internal 
implementation detail not visible to end users.

Kwok