drop unused ax_compare_version.m4

Its usage was gone at 6fc27b413d2252a45d2dbcb3dd8ca644ffb678da.
employ Check for unit tests

Now configure has option --enable-unittest to enable unit tests.
This contains test cases for ASTIndexNameNode, charBuffer, solverError, and util.
add test cases for sbmlsolver/sbml.h
make SBMLResults_free() ignore null pointer safely

Otherwise the following code got a segfault at the last line:
> SBMLResultsArray_t *ra;
> ra = SBMLResultsArray_allocate(1);
> SBMLResultsArray_free(ra);
This header contains libsbml's types
SBMLResultsMatrix_getResults() is not yet implemented
add unit test for sbmlsolver/sbmlResults.h
call for extra warnings
better include-guard name
function signatures contains libsbml's types
start replacing a deeper node than shallower one

Note that ASTNode_getListOfNodes() searches in a depth-first manner.
add test cases for sbmlsolver/modelSimplify.h
-Wsign-compare
-Wunused-parameter
findRoot requires an argument
require libsbml 3.4.1 or later
fix warning: 'parameterMode' may be used uninitialized in this function
fix a regression introduced at 383e38072aaa52c42ed3019f3459f2eaaf3df829
add test cases for AST_replaceConstants
DRY