Python interpreter and libraries/headers detection.

- The user can now pass its own interpreter.
- Development libraries and headers can be requested.
- Travis-CI switched to new container-based workers.

test/python_libs_custom/src/CMakeLists.txt

@@ -0,0 +1,2 @@
+add_executable(example example.cpp)
+target_link_libraries(example ${PYTHON_LIBRARIES})

test/python_libs_custom/src/example.cpp

@@ -0,0 +1,11 @@
+#include <iostream>
+#include <Python.h>
+
+int main(int argc, char *argv[])
+{
+    Py_SetProgramName(argv[0]);  /* optional but recommended */
+    Py_Initialize();
+    PyRun_SimpleString("print('PASSED')\n");
+    Py_Finalize();
+    return 0;
+}