Fix `cmake/GitTagVersion.cmake` to not throw errors when in a shallow
git clone, or any git clone that has no tags.
And when either git or git tag version info is not available, set the
version variables to the defaults, which are:
```
VERSION = '2.0.0'
REVISION = 'unknown'
VERSION_RELEASE = 0
```
To get the version, find the last tag in git tag of the form "v2.0.0" or
"2.0.0". If this is the last tag and the current commit matches the ref
of this tag, the revision will be empty.
In the case that the current commit is not tagged with a version, to get
the revision, use the short sha unless the current commit is tagged with
something that is not a version string, e.g. "feature-foo", in which
case the revision will be "feature-foo".
If the current commit is tagged as e.g. "v2.0.1-foo" or "2.0.1-foo" then
the version will be "2.0.1" and the revision will be "foo". Tags of this
form are also checked when finding the current version.
This is all done in cmake. If there is no git detected, the version will
be "2.0.0" and the revision will be "unknown".
gcc lto wrapper commands such as `gcc-ar` will segfault with some
versions of gcc 7.x when called via an absolute path, see:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80717
Generalize the mechanism for compiling a small C program on the host
(currently used for bin2c for Wx resources) and use it to wrap the gcc
wrappers in a little executable that prepends their dirname to PATH and
runs them with `execvp()`.
Make LTO default to ON again, except on win32 with gcc < 7, because
those toolchains produce broken binaries with LTO enabled.
Also add `-ffat-lto-objects` to compiler flags for gcc when LTO is
enabled, this will increase the chances that the LTO build will succeed
even if there are issues with the binutils wrappers. Clang does not
support this.
Rafael Kitover