meson.build: correct setting of GIT_EXEC_PATH

meson.build: correct setting of GIT_EXEC_PATH

For the non-'runtime prefix' case, the meson build sets the GIT_EXEC_PATH
build variable to an absolute path equivalent to <prefix>/libexec/git-core.
In comparison, the default make build sets it to a relative path equivalent
to 'libexec/git-core'. Indeed, the make build requires the use of some
means outside of the Makefile (eg. config.mak[.*] or the command-line)
to set GIT_EXEC_PATH to anything other than 'libexec/git-core'.

For example, the make invocation:

  $ make gitexecdir=/some/other/bin all install

will build git with GIT_EXEC_PATH set to '/some/other/bin' and install
the 'library' executables to that location. However, without setting the
'gitexecdir' make variable, irrespective of the 'runtime prefix' setting,
the GIT_EXEC_PATH is always set to 'libexec/git-core'.

The meson built-in 'libexecdir' option can be used to provide a similar
configurability. The default value for the option is 'libexec'. Attempting
to set the option to '' on the command-line, will reset it to the '.'
string, presumably to ensure a relative path value.

This commit allows the meson build, similar to the above, to configure the
project like:

  $ meson setup --buildtype=debugoptimized -Dprefix=$HOME -Dpcre2=disabled \
      -Dlibexecdir=/some/other/bin build

so that the GIT_EXEC_PATH is set to '/some/other/bin'. Absent the
-Dlibexecdir argument, the GIT_EXEC_PATH is set to 'libexec/git-core'.

In order to correct the value of GIT_EXEC_PATH, default the value to the
static string value 'libexec/git-core', and only override if the value
of the 'libexecdir' option has a value different to 'libexec' or '.'.
Also, like the Makefile, add a check for an absolute path when the
runtime prefix option is true (and if so, error out).

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>