Express version information in decimal

[argilo]

Fixes #720.

VOLK's version header (created in #346) unintentionally expresses version numbers in octal, because it prefixes the (decimal) numbers with zeroes. This will fail (with error: invalid digit "8" in octal constant) as soon as any component of the version number reaches 8.

The documentation in the header states that VOLK_VERSION is decimal, and this is a more convenient format, so I think it makes sense to switch to that. GNU radio already uses #if VOLK_VERSION >= 030100, but this condition will continue to be true if VOLK switches to decimal because 30100₁₀ is greater than 030100₈ (= 12352₁₀).

I considered instead changing the header documentation to indicate that the format is octal, but it would be difficult to implement correct header generation in CMake, because CMake's math only supports decimal and hexadecimal output formats.

[argilo]

Before:

#define VOLK_VERSION_MAJOR 03
#define VOLK_VERSION_MINOR 01
#define VOLK_VERSION_MAINT 00

[...]

#define VOLK_VERSION 030100

After:

#define VOLK_VERSION_MAJOR 3
#define VOLK_VERSION_MINOR 1
#define VOLK_VERSION_MAINT 0

[...]

#define VOLK_VERSION 30100

[argilo]

GNSS-SDR is the one other user of VOLK that I know of, and it doesn't use the VOLK_VERSION macros.

[marcusmueller]

Let's do this, before the current state causes any more damage.

[jdemel]

LGTM

[jdemel]

This code might be affected:
https://github.com/jj1bdx/airspy-fmradion/blob/c6ca61c600dc48f0e0af2322c61fe6ab16e25c3b/sfmbase/MultipathFilter.cpp#L139

[argilo]

This code might be affected

That will continue working, for the same reason that GNU Radio's checks will. Future decimal versions numbers will be greater than 030100₈.