gbuild: SOLARBINDIR does not exist any more

Change-Id: Ia158e398acbd5a48dca33d59dd47705376a4dc26
remove SOLARENV variable

It is constant and can just be replaced by $(SRCDIR)/solenv.

Use BUILD_TYPE where it was used to check if config_*.mk is sourced.

Change-Id: Ib9d480c57194b6340093aa47776f8768df69b7d1
gbuild: LinkTarget: setting FLAGS on clean target not necessary

Change-Id: Icc07e06e0ea7a7e5094d758a9aba0415449f706f
gbuild: gb_LinkTarget__get_headers_check can signal error now

... since no external link targets not with gbuild should be used via
gb_LinkTarget_use_library/static_library.

Change-Id: I1c3188dbbe2eaa2f5acfb8944a79ad3b67a95998
gbuild: LinkTarget: why are loads of variables set on headers_target

It is not obvious to me why that was ever needed.

Change-Id: I3df679b41f0685ab288294fa3fadfede093f6f69
gbuild: LinkTarget: remove unnecessary variables on dep_target

Since commit 8b5a984d45005d3df1c89eae897d6e04612625d8 it is no longer
the case that objects get build only via a dependency from
Linktarget's dep_target, relying on the variables set on dep_target;
now objects only get built via dependency from LinkTarget itself.

So there is no need to set these variables on dep_target any more.

Change-Id: Id0a831342ae6cfcff3bebe7010ece923f0262703
gbuild: Library: remove pointless functions

gb_Library_get_install_target and gb_Library__get_linktarget_target

Change-Id: I446e642fb43cf9804900b99b32dcc4c621dcb1c7
gbuild: StaticLibrary: remove gb_StaticLibrary__get_linktarget_target

Change-Id: I82ddbb3c82fa5f92e27a938a65a282ed67826a1e
gbuild: Executable: remove pointless indirection here too

gb_Executable__get_linktarget_target, gb_Executable_get_install_target
and gb_Executable_get_install_target_for_build

Change-Id: I1b0397c6982e885cd4834c2001264b1f6ae0a99c
gbuild: Library: remove gb_Library__get_final_target

... use gb_Library_get_exports_target instead, which is already a
"final" target for Library.

Change-Id: I5b8a708ba97e351ef383dcc64ead4ab9ea8705c7