mem: use memalign as fallback for posix_memalign and _aligned_malloc

posix_memalign is not available in Android <= 4.1 (API level 16 and
below). Fixes #140
9 jobs for master in 2 minutes and 59 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #32896
amd64 debian
build-debian

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passed #32900
debian aarch64
build-debian-aarch64

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passed #32901
debian clang5 aarch64
build-debian-aarch64-clang-5

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passed #32897
amd64 debian
build-debian-static

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passed #32903
debian aarch64
build-debian-werror

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passed #32902
macos
build-macos

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passed #32898
win32
build-win32

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passed #32899
win64
build-win64

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  Test
passed #32904
amd64 debian
test-debian

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