Add CPM_SOURCE_CACHE environmental variable support and keep existing sources (#83)

* read CPM_SOURCE_CACHE from environment

* update readme

* cleanup

* add cache tests

examples/build_all.py

@@ -0,0 +1,29 @@
+#!/usr/bin/python3
+
+from pathlib import Path
+from subprocess import PIPE, run
+
+examples = [
+  x for x in Path(__file__).parent.iterdir() if x.is_dir() and (x / 'CMakeLists.txt').exists()
+]
+
+assert(len(examples) > 0)
+
+def runCommand(command):
+  print('- %s' % command)
+  result = run(command, stdout=PIPE, stderr=PIPE, universal_newlines=True, shell=True)
+  if result.returncode != 0:
+    print("error while running '%s':\n" % command, '  ' + str(result.stderr).replace('\n','\n  '))
+    exit(result.returncode)
+  return result.stdout
+
+print('')
+for example in examples:
+  print("running example %s" % example.name)
+  print("================" + ('=' * len(example.name)))
+  project = Path(".") / 'build' / example.name
+  configure = runCommand('cmake -H%s -B%s' % (example, project))
+  print('  ' + '\n  '.join([line for line in configure.split('\n') if 'CPM:' in line]))
+  build = runCommand('cmake --build %s -j4' % (project))
+  print('  ' + '\n  '.join([line for line in build.split('\n') if 'Built target' in line]))
+  print('')