build-sys: Detect whether struct statx is defined in sys/stat.h

build-sys: Detect whether struct statx is defined in sys/stat.h

Starting with glibc 2.27.9000-36.fc29, include file sys/stat.h will have a
definition for struct statx, in which case include file linux/stat.h should be
avoided, in order to prevent a duplicate definition.

    In file included from ../src/basic/missing.h:18,
		     from ../src/basic/util.h:28,
		     from ../src/basic/hashmap.h:10,
		     from ../src/shared/bus-util.h:12,
		     from ../src/libsystemd/sd-bus/bus-creds.c:11:
    /usr/include/linux/stat.h:99:8: error: redefinition of ‘struct statx’
     struct statx {
	    ^~~~~
    In file included from /usr/include/sys/stat.h:446,
		     from ../src/basic/util.h:19,
		     from ../src/basic/hashmap.h:10,
		     from ../src/shared/bus-util.h:12,
		     from ../src/libsystemd/sd-bus/bus-creds.c:11:
    /usr/include/bits/statx.h:36:8: note: originally defined here
     struct statx
	    ^~~~~

Extend our meson.build to look for struct statx when only sys/stat.h is
included and, in that case, do not include linux/stat.h anymore.

Tested that systemd builds correctly when using a glibc version that includes a
definition for struct statx.

glibc Fedora RPM update:
https://src.fedoraproject.org/rpms/glibc/c/28cb5d31fc1e5887912283c889689c47076278ae

glibc upstream commit:
https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=fd70af45528d59a00eb3190ef6706cb299488fcd