debian: add support for DEB_BUILD_OPTIONS=parallel=N #97

Job	Run time
Build and test with ASAN enabled	1m 35s
Build and test with clang	1m 19s
Cross-compile for Android (aarch64)	45s
Cross-compile for Android (armv7a)	26s
Build and test with gcc	1m 0s
Build and test with gcc -m32	1m 40s
Cross-compile for Android (i686)	36s
Build and test on macOS	1m 17s
Cross-compile for Android (x86_64)	37s
Build and test with UBSAN enabled	1m 21s
Build mke2fs on Windows with mingw32	2m 23s
Build mke2fs on Windows with mingw64	2m 16s
	15m 15s

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