Previously, passing '-nostdinc -isystem $dir' only searched the given
directory but not any compiler-specific directories.
Discovered by fontconfig, which includes <stdatomic.h> from C11, which
lives in /usr/include/gcc-10 instead of /usr/include.
Change the preprocessor options to '--sysroot' instead, to align them
with how the compiler is invoked using build.sh.
e928d09a9d