add -std=c++11 as c++17 has deprecated dynamic exception specifications

Modern g++ (11.2) uses -std=c++17 as default, which doesn't permit
dynamic exception specifications anymore.  Downgrading to c++11 will
turn the related compiler errors into deprecation warnings:

 warning: dynamic exception specifications are deprecated in C++11

configure.ac

@@ -27,7 +27,7 @@ AC_CHECK_LIB(capi20,capi20_register,,AC_MSG_ERROR(libcapi20 not found))
 AC_CHECK_LIB(pthread,pthread_create,,AC_MSG_ERROR(libpthread not found))
 AM_PATH_PYTHON([2.7])
 PGAC_CHECK_PYTHON_EMBED_SETUP
-CPPFLAGS='-DLOCALSTATEDIR=\"$(localstatedir)\" -DPKGDATADIR=\"$(pkgdatadir)\" -DPKGSYSCONFDIR=\"$(sysconfdir)/capisuite\" -DPKGLIBDIR=\"$(pkglibdir)\" $(python_includespec)'
+CPPFLAGS='-std=c++11 -DLOCALSTATEDIR=\"$(localstatedir)\" -DPKGDATADIR=\"$(pkgdatadir)\" -DPKGSYSCONFDIR=\"$(sysconfdir)/capisuite\" -DPKGLIBDIR=\"$(pkglibdir)\" $(python_includespec)'
 
 AC_CONFIG_FILES([Makefile src/Makefile src/backend/Makefile src/modules/Makefile src/application/Makefile src/capisuite-py/Makefile scripts/Makefile scripts/waves/Makefile docs/Makefile scripts/capisuite.service])
 AC_OUTPUT