Fix

autoconf places a lot of " in the define string. this is to properly
escape them
Fixed conditional compilation with exceptions

Two fixes:
1. with autoconf, the proper macro for exceptions is __PLUMED_HAS_EXCEPTIONS
I fix it (breaking the compatibility with existing Makefile.conf)
because I think this is not used by anyone actually
2. uses macro __PLUMED_HAS_EXECINFO to decide whether execinfo.h should
be included or not