* Musl libc does not provide adaptive mutex. Added feature test for PTHREAD_MUTEX_ADAPTIVE_NP.

* Musl libc does not provide backtrace(3). Added a feature check for backtrace(3).

* Fixed compiler error.

* Musl libc does not implement backtrace(3). Added platform check for libexecinfo.

* Alpine does not appear to support gcc -pg option. By default (gcc has PIE option enabled) it fails with:

gcc: error: -pie and -pg|p|profile are incompatible when linking

When -fno-PIE and -nopie are used it fails with:

/usr/lib/gcc/x86_64-alpine-linux-musl/5.3.0/../../../../x86_64-alpine-linux-musl/bin/ld: cannot find gcrt1.o: No such file or directory

Added gcc -pg platform test and output PROFILING_FLAGS accordingly. Replaced pg var in Makefile with PROFILING_FLAGS.

* fix segfault when TEST_IOCTL_FRIENDLY_TMPDIR is undefined and default candidates are not suitable

* use ASSERT_DOUBLE_EQ instead of ASSERT_EQ

* When compiled with ROCKSDB_MALLOC_USABLE_SIZE UniversalCompactionFourPaths and UniversalCompactionSecondPathRatio tests fail due to premature memtable flushes on systems with 16-byte alignment. Arena runs out of block space before GenerateNewFile() completes.

Increased options.write_buffer_size.