[Ada] Fix bugs in gprmake

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on mainline.

There were several bugs in Build_Global_Archive, causing sometimes
unnecessary rebuilding of the global archive.
The test for this is, using two C only project files, both with source,
with no mains, project A importing project B:
  - compile the sources of B (gprmake -P b)
  - wait 10 seconds
  - invoke gprmake to compile the sources of A (gprmake -P a)
  - a subsequent invocation of gprmake to compile the sources of A
    should do nothing, inparticular it should no rebuild the global
    archive.

2005-07-04  Vincent Celier  <celier@adacore.com>

	* makegpr.adb (Build_Global_Archive): Make sure the list of sources is
	correctly computed and the main project data is not modified while
	doing so.
	(Add_C_Plus_Plus_Link_For_Gnatmake): Always link with the C++ compiler
	(Choose_C_Plus_Plus_Link_Process): Do not generate shell script
	c++linker as this does not work on some platforms.

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