configure -sysroot is not enough for cross compilation, one must also define CMAKE_SYSROOT

    PKG_CONFIG_PATH=  \
    PKG_CONFIG_SYSROOT_DIR="$SYSROOT" \
    PKG_CONFIG_LIBDIR="$SYSROOT/usr/lib/$TRIPLET/pkgconfig:$SYSROOT/usr/share/pkgconfig" \
        $SRC_DIR/configure \
        -make examples -xcb -opengl es2 -debug -linker lld \
        -sysroot "$SYSROOT" -prefix "$PREFIX" \
        -qt-host-path "$QT_HOST_PATH" \
        --  -DCMAKE_C_COMPILER="$CC" -DCMAKE_CXX_COMPILER="$CXX" \
            -DCMAKE_C_FLAGS="$CFLAGS" -DCMAKE_CXX_FLAGS="$CXXFLAGS" -DCMAKE_ASM_FLAGS="$ASMFLAGS" \
            -DCMAKE_SHARED_LINKER_FLAGS="$LDFLAGS" -DCMAKE_EXE_LINKER_FLAGS="$LDFLAGS" -DCMAKE_MODULE_LINKER_FLAGS="$LDFLAGS" \
            -DCMAKE_SYSROOT="$SYSROOT" -DCMAKE_SYSTEM_NAME=Linux \
            -DCMAKE_FIND_ROOT_PATH_MODE_PROGRAM=NEVER \
            -DCMAKE_FIND_ROOT_PATH_MODE_LIBRARY=ONLY -DCMAKE_FIND_ROOT_PATH_MODE_INCLUDE=ONLY -DCMAKE_FIND_ROOT_PATH_MODE_PACKAGE=ONLY

CMake Warning:
  Manually-specified variables were not used by the project:

    INPUT_sysroot