Before this change, autoconf was emitting the following warnings:
```
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_COMPILE_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
configure.ac:22: warning: AC_RUN_IFELSE was called before AC_USE_SYSTEM_EXTENSIONS
../../lib/autoconf/specific.m4:368: AC_USE_SYSTEM_EXTENSIONS is expanded from...
configure.ac:22: the top level
```
Not all the compilers that HarfBuzz is buildable on supports round() and
has the header stdbool.h, so we check for them and define HAVE_ROUND and
HAVE_STDBOOL_H repsectively in our CFLAGS so that we include them only
when they are found, or use fallback implementations when necessary.
Also include FindPythonInterp earlier as we need PYTHON_EXECUTABLE to be
set for running the tests.