Revert "targets/opencl: Link against libgallium.la instead of libgallium.a"

This reverts commit 4148a29ed83d1d85bff3d4e40e847128011c3f20.

This is a work-around for bug:
https://bugs.freedesktop.org/show_bug.cgi?id=59334

We really should be linking against libgallium.la instead of
libgallium.a, but until we can figure why linking against libgallium.la
causes runtime failures in clover we will continue to link against
libgallium.a

Acked-by: Andreas Boll <andreas.boll.dev@gmail.com>
Tested-by: Aaron Watry <awatry@gmail.com>
radeon, clover: Make it compile on LLVM >= r171366.
configure.ac: Save user {C,CXX}FLAGS and append them at end.

This way the user has the privilege of last decision and so the option to build an optimized debug build again.
Suppress clang's warnings about unused CFLAGS and CXXFLAGS.
gallium/auxiliary: Build libgallium shared.
Drop last parts of compatibility for the old Mesa build system.
mapi: Build libglapi always shared.
mesa: Build libmesa shared.
Install all internal shared libs to $(libdir)/mesa-$VERSION.
Makefile.am: s:-no-undefined:-Wl,--no-undefined to make it work.
gallium/drivers: Build libs -shared.
i915: Conditionally build an i915g driver instead of two i915 drivers.
configure: Build src/gtest at first.