Move the existing systemd sections over there.

release v0.21.20

Fixes strange C++11 compatibility problems during GTest installation:

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1249:1: error: unknown type name 'constexpr'
 constexpr bool TypedTestCaseIsDeprecated() { return true; }

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1249:11: error: expected unqualified-id
 constexpr bool TypedTestCaseIsDeprecated() { return true; }

 /tmp/gtest-20200216-20679-mu1044/googletest/include/gtest/internal/gtest-internal.h:1254:1: error: unknown type name 'constexpr'
 constexpr bool RegisterTypedTestCase_P_IsDeprecated() { return true; }

pip is bugged on arm64 and s390x. Add --no-cache-dir to fix. For
consistency, it doesn't hurt to add everywhere.

Closes https://github.com/MusicPlayerDaemon/MPD/pull/739 (essentially
the same, but with a proper explanation)

"The issue is that ParseCommandArgSignedSongTime parses with
SongTime::FromS, not SignedSongTime::FromS, before casting back to a
SignedSongTime for the return. With x86 overflow rules this doesn't
matter, but on ARM the first cast turns negative values to zero."

Closes https://github.com/MusicPlayerDaemon/MPD/issues/757

Get rid of the macro hell.

Merge remote-tracking branches 'neheb/patch-2', 'neheb/con', 'neheb/cons', 'neheb/guruhg', 'neheb/r12R3', 'neheb/fefgheh' and 'neheb/rhgerg3453'

Found with bugprone-move-forwarding-reference
Signed-off-by: Rosen Penev <rosenp@gmail.com>

The rhs evaluates to void *const, not const void*.

Found with misc-misplaced-const
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-static-definition-in-anonymous-namespace
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-uppercase-literal-suffix
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-redundant-control-flow
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-avoid-const-params-in-decls
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-const-return-type
Signed-off-by: Rosen Penev <rosenp@gmail.com>

pip is bugged on arm64 and s390x. Add --no-cache-dir to fix. For
consistency, it doesn't hurt to add everywhere.

This is still needed for GTest 1.8.0 (Travis / Ubuntu Bionic).

This doesn't work because IterableSplitString() returns its elements
by value.

Fixes clang warning:

 loop variable 'i' is always a copy because the range of type 'IterableSplitString' (aka 'BasicIterableSplitString<char>') does not return a reference [-Werror,-Wrange-loop-analysis]

Found with bugprone-suspicious-missing-comma
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with readability-string-compare
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Found with performance-unnecessary-value-param
Signed-off-by: Rosen Penev <rosenp@gmail.com>

Merge remote-tracking branches 'neheb/uniq', 'neheb/bool', 'neheb/loop', 'neheb/bool2', 'neheb/perf', 'neheb/void' and 'neheb/value'