IPO: implemented INTERPROCEDURAL_OPTIMIZATION for gcc (-flto)

Using the IPO specific rule variables in the Ninja generator should
fix intel IPO as well.
Merge topic 'gcc-ipo' into next

e4c5b620 IPO: implemented INTERPROCEDURAL_OPTIMIZATION for gcc (-flto)
Revert "IPO: implemented INTERPROCEDURAL_OPTIMIZATION for gcc (-flto)"

This reverts commit e4c5b620c7bc48c9145aa1fe9b23b6f97d68ea22.
Merge topic 'gcc-ipo' into next

d07f4302 Revert "IPO: implemented INTERPROCEDURAL_OPTIMIZATION for gcc (-flto)"
cmTarget: Don't try to get sources of an INTERFACE_LIBRARY.

An an assert to ensure this.
Merge topic 'INTERFACE-no-sources' into next

9db9c1fc cmTarget: Don't try to get sources of an INTERFACE_LIBRARY.