configure.ac: don't require libpython*.so in manylinux cibuildwheel

manylinux intentionally omits libpython*.so libraries from its build
containers to discourage/prevent wheels' native extension libraries from
linking against the Python DSO. We don't do that (anymore), but the
AX_PYTHON_DEVEL Autoconf macro expects to be able to find it and fails
its final sanity check if it can't. Thankfully, more recent versions of
the macro support an 'optional' flag that allows configure to proceed
even if the check fails. So now we set that flag and just check that we
have a value in $PYTHON_CPPFLAGS, which is all the SWIG_PYTHON macro
cares about in the first place.

Author: whitslack

configure.ac

@@ -311,7 +311,11 @@ AC_SUBST([libsecp256k1_LIBS])
 #
 # Python facilities
 #
-AX_PYTHON_DEVEL([>= '2.7.0'])
+dnl We set the 'optional' flag here because manylinux cibuildwheel containers
+dnl don't include libpython*.so, and thus AX_PYTHON_DEVEL fails its final sanity
+dnl check. We actually don't link against libpython*.so anyway, so all we care
+dnl about is that we get a good value of $PYTHON_CPPFLAGS to pass to SWIG.
+AX_PYTHON_DEVEL([>= '2.7.0'], [true])
 AM_CONDITIONAL([HAVE_PYTHON], [test "$PYTHON" != ""])
 
 
@@ -333,10 +337,10 @@ AC_ARG_ENABLE(swig-python,
     [swig_python=$enableval], [swig_python=no])
 AM_CONDITIONAL([USE_SWIG_PYTHON], [test "x$swig_python" = "xyes"])
 
-AM_CONDITIONAL([RUN_PYTHON_TESTS], [test "$PYTHON" != "" -a "x$pythonexists" = "xyes"])
+AM_CONDITIONAL([RUN_PYTHON_TESTS], [test -n "$PYTHON"])
 
 if test "x$swig_python" = "xyes"; then
-    if test "x$pythonexists" != "xyes"; then
+    if test -z "$PYTHON_CPPFLAGS"; then
         AC_MSG_FAILURE([ERROR: No usable Python was found for swig-python])
     fi
     if test "x$elements_abi" = "xno"; then