debian: add support for DEB_BUILD_OPTIONS=parallel=N #98

Job	Run time
Build and test with ASAN enabled	2m 57s
Build and test with clang	1m 13s
Cross-compile for Android (aarch64)	37s
Cross-compile for Android (armv7a)	36s
Build and test with gcc	57s
Cross-compile for Android (i686)	34s
Build and test with gcc -m32	1m 15s
Build and test on macOS	1m 26s
Cross-compile for Android (x86_64)	55s
Build and test with UBSAN enabled	1m 24s
Build mke2fs on Windows with mingw32	2m 20s
Build mke2fs on Windows with mingw64	2m 21s
	16m 35s

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