Configure: check for lex/yacc availability in path

Autoconf provides macros to check for `Particular programs'. Among
those are `lex' and `yacc' families of programs. When the relevant
macro is used, autoconf will look for a list of programs in the PATH,
perform execution check of the programs found, and set the associated
variable to the best possible match (for example, `flex' has precedence
over `lex').

The problem comes from the behavior of those two macros when no match
is found in the PATH: associated variables are set to a default value
and no further check is performed. This can lead to running ./configure
successfully and encountering an obfuscated error during compilation
for the output of `lex' and `yacc' could not be produced and fed to
the compiler.

This patch adds checks on the result of autoconf macros, and aborts
./configure execution if needed.

Signed-off-by: Clement Chauplannaz <chauplac@gmail.com>