cmake: Check for Argp also on Linux to fix alpine build

This adjusts also usage of ARGP_LIBRARY to use ARGP_LIBRARIES which is defined
by the FindArgp module, unlike the former one in case it is provided by libc
directly.

Fixes: #167

Signed-off-by: Jakub Jelen <jjelen@redhat.com>
Reviewed-by: Norbert Pocs <npocs@redhat.com>

cmake/Modules/FindArgp.cmake

@@ -1,4 +1,8 @@
 # - Try to find ARGP
+#
+# The argp can be either shipped as part of libc (ex. glibc) or as a separate
+# library that requires additional linking (ex. Windows, Mac, musl libc, ...)
+#
 # Once done this will define
 #
 #  ARGP_ROOT_DIR - Set this variable to the root installation of ARGP