Add configure option --enable-fixed-path.

* configure.ac: Add option --enable-fixed-path.
(FIXED_SEARCH_PATH): New ac_define.
* src/posix-util.c (walk_path): Make use of the option.  Remove
current directory from fallback PATH.

Note that using this option PATH is entirely ignored by GPGME and only
the given value is used instead to locate the GnuPG binaries.

On Android the use of PATH is not desirable. Instead the GnuPG tools
are expected in a fixed directory. By using

./configure --enable-fixed-path="/foo/bar"

gpg et al are expected to be installed as /foo/bar/gpg. With

./configure --enable-fixed-path="/foo/bar:/bin"

gpg is expected as /foo/bar/gpg or /bin/gpg.