Test if errno.h is available for some platforms

[Vincent Richomme]

For now I have commented everything in d\phobos2\config\errno.x3 and I 
am getting an other error about not finding gcc/config/fpcls.d:

make[3]: Entering directory 
`/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/arm-mingw32ce/libphobos'
true "AR_FLAGS=rc" "CC_FOR_BUILD=gcc" 
"CC_FOR_TARGET=/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/./gcc/xgcc 
-B/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/./gcc/ 
-B/opt/mingw32ce/arm-mingw32ce/bin/ -B/opt/mingw32ce/arm-mingw32ce/lib/ 
-isystem /opt/mingw32ce/arm-mingw32ce/include -isystem 
/opt/mingw32ce/arm-mingw32ce/sys-include" "CFLAGS=-O2 -g -O2 " 
"CXXFLAGS=-g -O2 " "CFLAGS_FOR_BUILD=-g -O2" "CFLAGS_FOR_TARGET=-O2 -g 
-O2 " "INSTALL=/usr/bin/install -c" "INSTALL_DATA=/usr/bin/install -c -m 
644" "INSTALL_PROGRAM=/usr/bin/install -c" 
"INSTALL_SCRIPT=/usr/bin/install -c" "LDFLAGS=" "LIBCFLAGS=-O2 -g -O2 " 
"LIBCFLAGS_FOR_TARGET=-O2 -g -O2 " "MAKE=make" "MAKEINFO=makeinfo 
--split-size=5000000 --split-size=5000000  " "PICFLAG=" 
"PICFLAG_FOR_TARGET=" "SHELL=/bin/sh" "RUNTESTFLAGS=" 
"exec_prefix=/opt/mingw32ce" "infodir=/opt/mingw32ce/info" 
"libdir=/opt/mingw32ce/lib" "includedir=/opt/mingw32ce/include" 
"prefix=/opt/mingw32ce" "tooldir=/opt/mingw32ce/arm-mingw32ce" 
"gdc_include_dir=/opt/mingw32ce/include/d2/4.1.0" "AR=arm-mingw32ce-ar" 
"AS=/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/./gcc/as" 
"LD=/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/./gcc/collect-ld" 
"RANLIB=arm-mingw32ce-ranlib" 
"NM=/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/./gcc/nm" 
"NM_FOR_BUILD=" "NM_FOR_TARGET=arm-mingw32ce-nm" "DESTDIR=" "WERROR=" 
DO=all multi-do # make
/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/./gcc/gdc 
-B/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/./gcc/ 
-B/opt/mingw32ce/arm-mingw32ce/bin/ -B/opt/mingw32ce/arm-mingw32ce/lib/ 
-isystem /opt/mingw32ce/arm-mingw32ce/include -isystem 
/opt/mingw32ce/arm-mingw32ce/sys-include -o std/boxer.o -g -frelease -O2 
  -nostdinc -pipe   \
	-I /d/Professionnel/Projets/cegcc/src/gcc/libphobos -I 
/d/Professionnel/Projets/cegcc/src/gcc/libphobos/internal/gc -I 
./arm-mingw32ce -c 
/d/Professionnel/Projets/cegcc/src/gcc/libphobos/std/boxer.d
/d/Professionnel/Projets/cegcc/src/gcc/libphobos/gcc/fpcls.d:19: module 
fpcls cannot read file 'gcc/config/fpcls.d'
make[3]: *** [std/boxer.o] Error 1
make[3]: Leaving directory 
`/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/arm-mingw32ce/libphobos'
make[2]: *** [all] Error 2
make[2]: Leaving directory 
`/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc/arm-mingw32ce/libphobos'
make[1]: *** [all-target-libphobos] Error 2
make[1]: Leaving directory 
`/d/Professionnel/Projets/cegcc/src/build-mingw32ce/gcc'
make: *** [all] Error 2