CPPHelper: warn when rebuild command cannot find C++ project source f… #1371

Job	Run time
android	25m 57s
flash-samples	40s
air-samples (4.0.5)	3m 29s
docs	32s
air-samples (4.1.5)	3m 1s
html5-samples	1m 28s
air-samples (4.2.5)	3m 9s
ios	13m 21s
air-samples (4.3.1)	3m 25s
linux	14m 39s
macos	31m 2s
windows	15m 13s
package-haxelib	1m 55s
hashlink-samples (windows-latest)	1m 2s
neko-samples (3.4.7, windows-latest)	1m 5s
hashlink-samples (ubuntu-20.04)	20s
hashlinkc-samples (windows-latest)	2m 2s
neko-samples (3.4.7, ubuntu-20.04)	17s
hashlink-samples (macos-13)	45s
hashlinkc-samples (ubuntu-20.04)	1m 35s
neko-samples (3.4.7, macos-13)	53s
hashlinkc-samples (macos-13)	1m 47s
neko-samples (4.2.5, windows-latest)	53s
neko-samples (4.2.5, ubuntu-20.04)	19s
neko-samples (4.2.5, macos-13)	48s
notify	2s
	2h 9m 39s

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