cross compile support

[liamstask]

Hi - I was attempting to cross compile (via buildroot) colm as a dependency for ragel, itself a dependency of another project, and ran into the following error during configure:

checking for ./DIST... configure: error: cannot check for file existence when cross compiling

in the meantime, i'm just patching this locally to avoid this specific check in order to determine whether to build the manual, but would be nice to support this.

configure env details:

``` PATH="host/bin:host/sbin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin" AR="host/bin/x86_64-buildroot-linux-gnu-ar" AS="host/bin/x86_64-buildroot-linux-gnu-as" LD="host/bin/x86_64-buildroot-linux-gnu-ld" NM="host/bin/x86_64-buildroot-linux-gnu-nm" CC="host/bin/x86_64-buildroot-linux-gnu-gcc" GCC="host/bin/x86_64-buildroot-linux-gnu-gcc" CPP="host/bin/x86_64-buildroot-linux-gnu-cpp" CXX="host/bin/x86_64-buildroot-linux-gnu-g++" FC="host/bin/x86_64-buildroot-linux-gnu-gfortran" F77="host/bin/x86_64-buildroot-linux-gnu-gfortran" RANLIB="host/bin/x86_64-buildroot-linux-gnu-ranlib" READELF="host/bin/x86_64-buildroot-linux-gnu-readelf" STRIP="host/bin/x86_64-buildroot-linux-gnu-strip" OBJCOPY="host/bin/x86_64-buildroot-linux-gnu-objcopy" OBJDUMP="host/bin/x86_64-buildroot-linux-gnu-objdump" AR_FOR_BUILD="/usr/bin/ar" AS_FOR_BUILD="/usr/bin/as" CC_FOR_BUILD="host/bin/ccache /usr/bin/gcc" GCC_FOR_BUILD="host/bin/ccache /usr/bin/gcc" CXX_FOR_BUILD="host/bin/ccache /usr/bin/g++" LD_FOR_BUILD="/usr/bin/ld" CPPFLAGS_FOR_BUILD="-Ihost/include" CFLAGS_FOR_BUILD="-O2 -Ihost/include" CXXFLAGS_FOR_BUILD="-O2 -Ihost/include" LDFLAGS_FOR_BUILD="-Lhost/lib -Wl,-rpath,host/lib" FCFLAGS_FOR_BUILD="" DEFAULT_ASSEMBLER="host/bin/x86_64-buildroot-linux-gnu-as" DEFAULT_LINKER="host/bin/x86_64-buildroot-linux-gnu-ld" CPPFLAGS="-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64" CFLAGS="-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os " CXXFLAGS="-D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os " LDFLAGS="" FCFLAGS=" -Os " FFLAGS=" -Os " PKG_CONFIG="host/bin/pkg-config" STAGING_DIR="host/x86_64-buildroot-linux-gnu/sysroot" INTLTOOL_PERL=/usr/bin/perl ac_cv_lbl_unaligned_fail=no ac_cv_func_mmap_fixed_mapped=yes ac_cv_func_memcmp_working=yes ac_cv_have_decl_malloc=yes gl_cv_func_malloc_0_nonnull=yes ac_cv_func_malloc_0_nonnull=yes ac_cv_func_calloc_0_nonnull=yes ac_cv_func_realloc_0_nonnull=yes lt_cv_sys_lib_search_path_spec="" ac_cv_c_bigendian=no CONFIG_SITE=/dev/null ./configure --target=x86_64-buildroot-linux-gnu --host=x86_64-buildroot-linux-gnu --build=x86_64-pc-linux-gnu --prefix=/usr --exec-prefix=/usr --sysconfdir=/etc --localstatedir=/var --program-prefix="" --disable-gtk-doc --disable-gtk-doc-html --disable-doc --disable-docs --disable-documentation --with-xmlto=no --with-fop=no --disable-dependency-tracking --enable-ipv6 --disable-nls --disable-static --enable-shared --disable-manual ```

[adrian-thurston]

Thanks @liamstask. That was always a bit of a poor attempt at determining if we are configuring from a distribution tarball or directly from source code control. There is probably a better way.

[Grommish]

I created a PR to correct this. https://github.com/adrian-thurston/colm/pull/124