As GDCFLAGS is overriden by the top-level make file with '-O2 -g',
libphobos ends up always being built with all contracts, invariants, and
asserts compiled in.  This adds a new configurable that defaults to omit
compiling any run-time checks into the library using '-frelease'.

Other choices either set the flags '-fno-release', enabling all run-time
checks, or '-fassert', which only compiles in asserts.

The omission of compiling in contracts results in a smaller library
size, with faster build times.

libphobos/ChangeLog:

	PR d/94305
	* Makefile.in: Regenerate.
	* configure: Regenerate.
	* configure.ac: Add --enable-libphobos-checking and substitute
	CHECKING_DFLAGS.  Remove -frelease from GDCFLAGS.
	* libdruntime/Makefile.am: Add CHECKING_DFLAGS to AM_DFLAGS.
	* libdruntime/Makefile.in: Regenerate.
	* src/Makefile.am: Add CHECKING_DFLAGS to AM_DFLAGS.
	* src/Makefile.in: Regenerate.
	* testsuite/Makefile.in: Regenerate.
	* testsuite/testsuite_flags.in: Add -fno-release -funittest to
	--gdcflags.