fix build issues with flex ver 2.5

fix build issues with flex ver 2.5

When building on an old environment, the flex generated
tc/emp_ematch.lex.c file would not compile.  The error given was:

emp_ematch.lex.c:1686: error: expected â\80\98;â\80\99, â\80\98,â\80\99 or â\80\98)â\80\99 before numeric constant

The emp_ematch.l uses 'str' as a start symbol name, and  flex would create
a '#define str 1' statement.  This particular version of flex,
unfortunately, used 'str' as names of string variables in the generated
parser functions.  This is line 1686 in the generated file:

YY_BUFFER_STATE ematch__scan_string (yyconst char * str )

This patch just substitutes 'str' for 'lexstr' in emp_ematch.l to avoid
the collision.