add -static-libgcc -static-libstdc++ for test executables when using mingw
fix GetExecutablePath test for mingw
Merge pull request #11 from tkelman/testexes-stdlib-static

Link test exes statically to libgcc, libstdc++ for MinGW
Build shared version of asl if BUILD_SHARED_LIBS is on.
Merge branch 'master' of github.com:ampl/mp
add mingw cross-compile to travis matrix