build: Fix AX_PROG_{CC,CXX}_FOR_BUILD macros

build: Fix AX_PROG_{CC,CXX}_FOR_BUILD macros

Override the cross_compiling and ac_tool_prefix variables by reassigning
to them instead of redefining the macros. Redefining them will actually
cause the variable names to be replaced instead of their content.

Furthermore push the definition of CPPFLAGS before running the checks
for the build tools to avoid the host CPPFLAGS from leaking into the
build CPPFLAGS.

While at it drop the redefinition of AC_TRY_COMPILER which hasn't been
used since autoconf 2.50 and make sure that all definitions are properly
popped when done (LDFLAGS, ac_cv_prog_CPP, ac_cv_prog_CXXCPP).

Acked-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>