Thread-safe initialization commit breaks OpenBSD build

[Mno-hime]

After 9159bc125bfb8989a5dbc2ce103312c2aa8906f5 build fails on OpenBSD 6.9 with:

  CC       libeatmydata/la-libeatmydata.lo
libeatmydata/libeatmydata.c:55: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'int'
libeatmydata/libeatmydata.c:56: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'int'
libeatmydata/libeatmydata.c:57: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'libc_open_t'
libeatmydata/libeatmydata.c:61: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'libc_fsync_t'
libeatmydata/libeatmydata.c:62: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'libc_sync_t'
libeatmydata/libeatmydata.c:63: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'libc_fdatasync_t'
libeatmydata/libeatmydata.c:64: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'libc_msync_t'
libeatmydata/libeatmydata.c: In function 'eatmydata_init':
libeatmydata/libeatmydata.c:94: error: 'init_running' undeclared (first use in this function)
libeatmydata/libeatmydata.c:94: error: (Each undeclared identifier is reported only once
libeatmydata/libeatmydata.c:94: error: for each function it appears in.)
libeatmydata/libeatmydata.c:95: error: 'libc_open' undeclared (first use in this function)
libeatmydata/libeatmydata.c:99: error: 'libc_fsync' undeclared (first use in this function)
libeatmydata/libeatmydata.c:100: error: 'libc_sync' undeclared (first use in this function)
libeatmydata/libeatmydata.c:101: error: 'libc_fdatasync' undeclared (first use in this function)
libeatmydata/libeatmydata.c:102: error: 'libc_msync' undeclared (first use in this function)
libeatmydata/libeatmydata.c:110: error: 'init_complete' undeclared (first use in this function)
libeatmydata/libeatmydata.c: In function 'eatmydata_is_hungry':
libeatmydata/libeatmydata.c:116: error: 'init_complete' undeclared (first use in this function)
libeatmydata/libeatmydata.c: In function 'fsync':
libeatmydata/libeatmydata.c:147: error: 'libc_fsync' undeclared (first use in this function)
libeatmydata/libeatmydata.c: In function 'sync':
libeatmydata/libeatmydata.c:157: error: 'libc_sync' undeclared (first use in this function)
libeatmydata/libeatmydata.c: In function 'open':
libeatmydata/libeatmydata.c:175: error: 'init_running' undeclared (first use in this function)
libeatmydata/libeatmydata.c:183: error: 'libc_open' undeclared (first use in this function)
libeatmydata/libeatmydata.c: In function 'fdatasync':
libeatmydata/libeatmydata.c:237: error: 'libc_fdatasync' undeclared (first use in this function)
libeatmydata/libeatmydata.c: In function 'msync':
libeatmydata/libeatmydata.c:249: error: 'libc_msync' undeclared (first use in this function)
*** Error 1 in . (Makefile:1027 'libeatmydata/la-libeatmydata.lo': @echo "  CC      " libeatmydata/la-libeatmydata.lo;/bin/sh ./libtool --si...)
*** Error 2 in /usr/src/libeatmydata-129 (Makefile:674 'all')