When CFLAGS is set to any value the configuration of libffi is broken. Examples: CFLAGS= vs. CFLAGS=-nologo 2206c2206 < checking command to parse dumpbin -symbols output from /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 object... ok --- > checking command to parse dumpbin -symbols output from /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 object... failed 2223,2224c2223,2225 < checking if /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 supports -c -o file.obj... yes < checking if /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 supports -c -o file.obj... (cached) yes --- > checking if /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 supports -c -o file.obj... no > checking if /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 supports -c -o file.obj... (cached) no It seems that having CFLAGS set causes "-Fd$(COMPILE_PDBFILE)" to be added to CFLAGS, yet COMPILE_PDBFILE is not defined anywhere (and anyway, that syntax is for make files). The result is this, from ~/js/src/ctypes/libffi/config.log: configure:8214: checking if /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 supports -c -o file.obj configure:8235: /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh -m64 -c -nologo -TC -nologo -W3 -Gy -Fd$(COMPILE_PDBFILE) -o out/conftest2.obj conftest.c >&5 cl -MD -nologo -W3 -c -nologo -TC -nologo -Gy -Fd$(COMPILE_PDBFILE) -Foout/conftest2.obj -Fdout/conftest2 -Fpout/conftest2 -Faout/conftest2 conftest.c/msvcc.sh: line 195: COMPILE_PDBFILE: command not found conftest.c /c/mozbuild/mozilla/js/src/ctypes/libffi/msvcc.sh: line 195: COMPILE_PDBFILE: command not found cl : Command line warning D9025 : overriding '/Fd' with '/Fdout/conftest2' configure:8239: $? = 0ng for C compiler default output file name configure:8261: result: nonftest.exe Seen with VS2008/SPI, Win SDK v7.1, on WinXP x64.