build: Don't cross-compile GLSL builtin compiler

build: Don't cross-compile GLSL builtin compiler

The builtin_compiler binary is used during the build process to generate
code for the builtin GLSL functions. Since this binary needs to be run
on the build host, it must not be cross-compiled.

This patch fixes the build system to compile a second version of the
source files and the builtin_compiler binary itself for the build
system. It does so by defining the CC_FOR_BUILD and CXX_FOR_BUILD
variables, which are searched for by the configure script and point to
the location of native C and C++ compilers.

In order for this to work properly, builtin_function.cpp is removed
from BUILT_SOURCES, otherwise the build system would try to generate it
before having had a chance to descend into the builtin_compiler
subdirectory. With the builtin_compiler and glsl_compiler now being
generated at different stages, the build instructions for glsl_compiler
can be simplified a bit.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>